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I made this chart where I kept the Population of NY as a constant.  Ie, This is the populations of States as a percentage of NY's population over time:

I've included (I don't think I missed any) every state that was in the top-15 in population in any census from 1880-2020.  

California:
In the late 1800s and early 1900s the "big four" states were NY, PA, IL, and OH.  Back then, California's population was a fraction of even the least populous of those and it didn't really even start gaining until after the turn of the century.  

In 1900 California's population was a shade under 1.5M which was:

• roughly 1/3 of Ohio's population of 4.2M
• less than 1/3 of Illinois' population of 4.8M
• less than 1/4 of Pennsylvania's population of 6.3M
• roughly 1/5 of New York's population of 7.3M

From then through 1940 California was gaining on the "big four" but only gradually compared to what was to come.  In 1940 on the eve of US involvement in WWII California's population was still (barely) less than Ohio's, substantially less than Illinois and Pennsylvania and barely over 1/2 of New York's population.  

Starting in 1940 California's population just exploded.  They passed OH, IL, and PA in the 1950 census and moved ahead of NY into #1 twenty years later.  The VERY rapid growth continued through 1990 then slowed and in the most recent decade (2010-2020) California's growth slowed to barely faster than NY, PA, IL, OH, etc.  

A lot of this, I think, is a result of the war and it's aftermath.  California's economy for many years was tightly tied to the defense industry both through the basing of large numbers of troops there and a lot of defense contractors having operations there.  

Air Conditioning was less of a factor for California than TX/FL/AZ because coastal California's temperatures are moderated by the relatively cool Pacific Ocean and, at least until relatively recently, the vast majority of California's population lived in coastal areas.  

Florida:
I think Florida is probably more A/C dependent than any of the states listed previously.  Prior to A/C I think that Florida was fine to visit in the winter but not somewhere a lot of people wanted to stay.  

Their population grew marginally faster than most states up through 1970ish then just exploded.  In the 1970 census their population of 6.8M was:

• Less than NJ and MI
• Nearly 4M less than OH (10.7M)
• Nearly 4.5M less than IL (11.1M)
• Barely over 1/2 of PA (11.8M)
• Barely over 1/3 of NY (18.2M)

In the half-century since their population grew to #3 behind only CA and TX.  

Looking back it amazes me that the first time I visited FL (1979) I travelled from a more populous state (OH) to a less populous state (FL).  

@OrangeAfroMan this might help give some context to the discussion we had about the Rose Bowl and the Coliseum. One can in hindsight look back and see them as failures because they weren't able to fill them the way you think they should... 

But in the context of the Roaring 20's, in a state that was rapidly growing in population, and that looked like everything was going to be "up and to the right" forever... I understand why the people behind building those stadiums perhaps were dreaming big. 

Perhaps it didn't pan out, but I understand why they thought it would. 

Pennsylvania:
This one really interests me because I can't come up with an obvious explanation.  If anyone has one or a theory, I'd be interested to hear it. 

From 1880-1920 Pennsylvania was not only the second most populous state but they were also a LOT closer to #1 NY than they were to #3 (IL) and #4 (OH).  Since then their population relative to the others has drifted downward considerably. 

I understand or at least I think I understand why CA, TX, and FL have grown faster than PA but I'm at a loss to explain why NY, IL, and OH have grown faster.  In 1920 PA was almost as populous as NY and substantially ahead of IL and OH.  A century later their population is roughly 2/3 of NY's, only barely over IL, and not all that far above OH.  Why? 

I can't necessarily explain OH. But NY and IL make sense...

If you think of the gradual urbanization of the US during that time, there was growth of certain "dominant" cities. New York and Chicago are two of those. DC is obviously one, Atlanta is another, I'd say Dallas/Houston both qualify, LA/SF/SD, Seattle, Boston and Denver might round out the list. Dominant cities are attractors of population at the regional (and sometimes even national) level, not just state level. 

I don't see Pittsburgh or Philly as "dominant" cities. So they might attract rural Pennsylvanians who were migrating to the city, but rural Pennsylvanians might also get pulled away by NY or DC... Or even Chicago. If they stayed in the area at all--they might also move South or West. 

But the dominant cities draw from beyond their own state's rural population. Think of Atlanta... It's not just the hub for Georgians that were moving to "the big city", but they'll pull from AL/MS/SC/TN/etc. Chicago will pull from the entire Midwest, not just IL. Denver pulls from the entire Mountain West, not just CO. Seattle from the entire Northwest, not just WA. And the California cities, quite frankly, pull nationally, just as NY or DC would.

I look at it like the way I look at Purdue & Indiana football. Indiana football kids grow up dreaming of playing for ND... Or UM or OSU... OH kids dream of playing for OSU. MI kids dream of playing for UM. So Purdue & Indiana have trouble even keeping their own kids in state, and much MORE trouble pulling from neighboring states, because nobody in Michigan or Ohio State is really all that excited about a Purdue or IU offer. 

I think Pennsylvania might be like that. Pittsburgh and Philadelphia are pretty solid cities. They're going to end up with a lot of the "in state recruits" so to speak. But they're going to have a hard time keeping NY or DC or Chicago (or LA) from poaching "recruits", and a VERY hard job pulling them from states outside PA. 

We moved to ATL in 1964.  I recall most of the houses we looked at had central AC, and it was a big factor even though the house we bought (new) did not.  We had a large window unit in the family room and a whole house fan.  My parents installed central air in 1970.  I think by 1970 every new middle class house had central air.

There is a residential area just north of me that was started in 1904.  Nearly every house there has a large front porch, which was their version of AC along with fans.  It's now a rather expensive area so they all have AC today.  Some of the houses have  been demo'd and replaced with new, most were just restored/updated.  They look really nice.

Living here without central air, even today in June, would be ... unpleasant.  I was just out running and I'm soaked.  There are other factors for population growth, in the 1950s there were a lot of cloth mills around, carpet production, etc., because labor was cheap, so was land.  The textile industry then moved to Asia leaving some towns here bereft.

But I think without AC,  the South would have half its current population.

I broadly viewed Pennsylvania to be heavily into heavy industry back in the day, Ohio less so save Cleveland/Akron/etc.

I dimly recall that ca. 1880 Cincinnati was the fourth largest city in the country.  I wouldn't like to survive their summers with AC either.