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I'll say right up front that this post may reveal my general ignorance of how image formats and compression works ... so with that in mind , here is the issue I'm trying to understand:

I have two .mov files to import into TVPaint . Both movie files have full color images (not line art) and the resolution of both movie files is 4096 x 3072 , compression is Apple PNG (with transparency, so the two movie layers can be combined into a composite image in TVPaint and have more layers added ).

The first movie file is 400 MB , the second movie file is 176 MB.

In my simple cartoonist's brain I would expect 400 MB of data + 176 MB of data = 576 MB file size ... but after the two movie files are imported to TVPaint and I save the project, the .tvpp file size is a huge 3.44 GB . (and yes I have TVPP compression enabled in the Preferences)

I tried to lighten the file size by eliminating as many extraneous images as possible : for example , where the animation may HOLD for 40 frames those are 40 full-color rendered frames on the imported movie , so I changed it to eliminate 39 of those frames and just hold the remaining one frame for 40 frames . This happens in several places during the animation, so I was able to eliminate many extra frames . However , after doing so and saving the file it was still showing as 1.4 GB , which is more than the original data of the two movie files (400 MB and 176 MB) .

I'm wondering why the size of the data increases so much when the movie files are imported into TVPaint ? Is there an option when importing that I should enable to make the file size smaller ?

Now , here's something strange (but good for me, so I'm not complaining !) : After I closed the file and went to re-open it later , worked on it a bit more (adding some additional animation layers) , I saved it again and now I notice that the file size is showing as 636 MB , which is closer to the original amount of data of the two movie files (400 MB + 176 MB ) . So what changed when I saved it before and it was 1.4 GB , then when I re-open it , work on it some more and save again it is now 636 MB ?

I can’t help you with your actual question about what changed (TVP is still such a wonderful terra incognita to me in those areas), but I can tell you that your initial idea that two files of certain size should amount to their sum as TVP project was incorrect in theory because those files were both compressed as files in themselves (with whatever settings the initial PNG had), and when you brought them to TVP they became uncompressed sequence of images.

TVP render could make the small again, when the program compresses your output footage again, but not the project in itself, I’d think. But clearly I’m wrong because it didn’t

Undo levels. TVP has a lot of them. Ever noticed how fast you can go backwards (ctrl-Z) and forwards again even when you've merged all your layers? That's because each state is stored somewhere.

Yes, files get large after you've imported a movie. I don't care, I know they'll get smaller again when I've worked a bit with the file.