> And I maintain that the difference cannot be explained by images and font cache alone.

It can't be explained by images and fonts alone, I agree, but images and fonts alone are still enormous compared to 70kB. On my screen an "e" on this site is 10*11 pixels. Subpixel positioning means we need at least 6 copies of each letter, subpixel antialiasing means we need three channel alpha (3 bytes per pixel), we have variations of bold, italic and bold-italic, 95 printable letters in ascii, that's a low-ball of 750kB for one size of one font. This comment page alone has 4 different sizes, I'd expect a chat app to have a few more.