I have noticed that images in .prc ebooks are almost always smaller than in other formats (e.g. .lit). In principle, MobiPocket has relatively good support for images see:
Image support and display. This page only mentions GIFs, but BMPs, PNGs and (recently) JPGs are also supported. The problem seems to be:
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Please also notice that there is a 64KB internal limitation for images (this is a restriction of the Mobipocket .PRC format). GIFs have to be smaller than 64KB.
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The limit is actually a bit less than 64KB, but in any case this file size is reached by relatively smalls GIFs.
My favorite test book is
Ring of Fire by Eric Flint from the Baen Free Library. I enclose screenshots from the .lit and .prc versions using FBReader under Windows (.prc under MobiPocket Reader looks similar to FBReader). The cover image is much smaller in the .prc file (a 40 KB 164x240 BMP) than the .lit file (a 65 KB 510x680 JPG). The last chapter includes a map, and it is again much smaller in the .prc (49 KB 293x159 BMP) than the .lit (56 KB 576x312 JPG). Note that the full size JPG map is less than 64KB, but JPG support is new in MobiPocket and I'm not sure how many publishers are using it (for backward compatibility).
I understand that .prc puts MobiPocket in a bit of a box with its record size limit, but Plucker can support large images in .pdb files. The mobigen command line converter automatically shrinks images, but that is why .prc ends up with smaller images. It does now have a -jpeg option, which helps if publishers are willing to drop support for old reader software.
The small image size might be ok if the MobiPocket Reader (and FBReader) autoscaled to larger sizes. There would be no new information in the larger image, but it would be easier to see.