Help! Export to PNG files TOO HUGE. Animate 2

When I exported my frames as PNG files, and they were HUGE. I’m guessing I did something wrong. I have 3696 png files in the frames folder, and they are 390 GB in size. Is this normal? 960 x 540 (72 dpi)

I created the characters in photoshop, then imported them with an alpha channel (because they were getting messed up on the render without the alpha). Perhaps I need to flatten everything before I output or something?

The last job I did was 5040 frames, and only 7.4 GB. 1920 x 1080 (72dpi)

What am I doing wrong? I tried to import the png files into Adobe Premiere (so I can add the sound and publish for youtube) but it is all too gigantic.

Also, do I need to keep those TGA files around once the project is complete? What are they for, preview only? Thanks.