Not sure if this helps any, but IIRC black takes the most size, right? Anyway:
No, it's compressed data.
For GIF the compression is runline, meaning that if you have a vertical gradient (where the colors change from top to bottom), the compression would be excellent, since each horizontal line would be the same color value. Take that same gradient and rotate it 90 degrees, to where the gradient runs from side to side, and the filesize will increase noticeably.
JPG compression is lossy (doesn't retain the original image information) is based on blocks of 8x8 pixels, where it divides the image up into 8x8 blocks, and - depending on the level of compression - begins to omit smaller details in that block.
PNG I'm not so sure about, but 8-bit PNG's have limited color tables (up to 256 colors, like GIF), while 24-bit PNGs have full-color (8-bit for each of red, green, and blue) plus 8 bits of alpha transparency.
So the largest GIF would probably be one where there are no repeated colors in any individual line. The largest JPG would probably be one where the quality level (inverse of compression) was set to 100, and the image was pixel-sized static. The largest PNG is anybody's guess.

EDIT: Here's some of the largest images I can come up with without going overboard in trying.

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