Why am I getting different sizes (MB) when exporting from the same file.

I have been using one file to create splash pages. The canvas size is 1980 x 1080 and I save them as pngs.  When I first created the file and made the first splash, I saved it as a png that was 957K. 
The second splash (with the same paramenters) was 1.4 MB
The 3rd splash was 1.7MB
The 4th was 2.1 MB
They were getting better every time I created a new one. Despite the fact that it wasn't changing in size. Color wise or what was being shown was also not getting more complicated, so visually it didn't make sense either. 

I thought maybe it was the file that was getting large and it was getting extra (info?) during the save. But I could still export my first splash and it still comes out at its original size (957K) 

I've tried taking the 4th splash page into a new file (just itself and even merged/flattened the layers) and now, that image is still coming out as over 2 MB. 

I have tried the export option/save for web. Its a little smaller, but still doesn't solve why each new splash I create in the file comes out as a larger png. 

Any ideas?