Originally Posted by DSpider
There's also the issue with embedded fonts. Acrobat can subset glyphs from a font for PDF files, whereas ePub files will contain the entire font. This means that if you're only using one character from a 600 KB font file, the entire font file will be embedded, where it would only take 6-10 KB in a subset. And if you use 10 different 600 KB font families... You get the idea.
But in general, images usually take up the most amount of space.
Calibre allows font subsetting. It works very well and can reduce the size of the embedded fonts significantly. Also, if you have fonts embedded that do not get used, they get removed for an even greater size reduction.
What I do is take the finished ePub, load it into Calibre, convert to ePub using subsetting and then I take the font directory out of the converted ePub, replace the font directory in the finished ePub, fix the CSS/OPF as needed to match the fonts and done. Now I have an much smaller ePub with subsetted fonts.