I don't notice the file size of the stories I download, and I've never checked the percentage of that space devoted to covers. All of my ereaders allow SD/microSD expansion, and these cards are cheap. My entire Calibre library amounts to 1.5 GB and I'm still using the 2GB cards I started with. I have some 4GB cards I can move up to, if and when I need to. These days the smallest cards that are readily available are probably 8GB and go for $5 (US) each -- and the 16GB cards start at about $8.
Basically, I don't think that the cover size is a factor for most readers.
|