I figured out the problem re the images showing up smaller in portrait mode in iBooks! My original scans of the two pages of sheet music were at 300dpi, and then I cropped those two pages down to individual measures, which gave me 9 separate PSD images (still at 300dpi). Then I brought those into Illustrator, did an "auto trace" on them (using the grayscale option) and then saved them as SVG -- but I guess that resolution info gets embedded in the SVG file.
Just as an experiment, I re-did the first image, changing the resolution to 72dpi, switched that one for the 300dpi one in my book, and now it stretches to the full width of the page.
Interesting that iBooks pays attention to image resolution, and displays it accordingly! I guess I'd just assumed that, as with web pages, even if you insert a higher-res image it would still just display it "by pixels", essentially ignoring any different resolution.
Anyway, that answers that -- although I'm still wondering why it's better to use SVG instead of JPEG or PNG. With SVG, isn't there more of a risk that the image format won't be supported by some device or other out there? I really don't know much (if anything) about SVG, never worked with them until now.
