I suspect that the graphics are being sized to the resolution of the Palm/Pocket PC screen that BD "thinks" the file is being created for, so I'm finishing up with a 320x240 (or whatever) graphic even through I'm viewing it on a 1024x768 screen
.
Strangly, though, it doesn't seem to be a "universal" problem. The "full screen" illustrations I've converted have gone really, really small, but the much smaller graphics in the "Dancing Men" story in the Sherlock Holmes omnibus have come out OK.