Ashley, those are the image file sizes, the C2 size for the player animation frames would be 262kb per frame (256px * 256 px * 4). There are approximately 1000 player/enemy animation frames in total which works out to 260Mb if I'm not mistaken.
If I add in 300 frames for environment objects per layout, say an average size of 512x512 (some larger, most smaller) that adds up to 300Mb, so rounding up I would expect minimum image memory usage of 600Mb (not including space required for tiled backgrounds which are also used).
The "ImageMemoryUsage" parameter in C2 varies between 600Mb and 1Gb which seems about right, with I assume the other 1.7Gb (of the 2.7Gb total) being taken up by the code/nodewebkit processes. There is a lot of room for improvement in the code, but I'd be surprised if I could claw back another 1Gb. I can't run the game in a browser any more (too big I assume) so I'm not able to use the layout debugger to troubleshoot.
I don't think there's much I can do to reduce image memory usage unless I reduce sprite sizes and/or reduce the number of frames.
Example frames for the player and environment in links below. Edit; Don't have the rep to post links yet.
TiAm, the game is PC only, so I'm aiming for around 2Gb total memory usage, I estimate half of this will be taken up by image memory, but given I have yet to add audio, I will probably struggle to stay within 2Gb.