Exported PNG size is different between identical PSD files

Hello,

First time in years of using Photoshop, I feel like an absolute idiot because I don't know what is happening...

I made a rather simple social post advertising a product in 8bit RGB 1080x1080px at 300ppi and when I saved it as PNG, it weighed 7.47mb... I thought that maybe I used some kind of big smart objects, but couldn't find anything, and out of frustration, I deleted all the layers and saved a white color fill PNG, which still was a whopping 6.59mb...

Then I created a new file with the same settings of 8bit RGB 1080x1080px at 300ppi and dragged all the layers from the first file to this new one, and guess what... saved PNG was 0.9mb...

So my question is... WHAT I AM MISSING?

Here are Image Size settings of both files and as you can see, they are identical