If I make Quintus into Book Lite the photos stay big. If I make Book Lite into Quintus they stay small.
1. As Jennifer said, this isn't theme dependent: it depends on what option you select when you insert the images and on how wide the originals are. If you select the "full size" option, and the original is wider than the main column of the theme, the inserted version will be as wide as possible (= as wide as the main column).
I'd like the same size photos in both
2. If by "same size" you mean as big as possible, see #1. If you mean exactly the same size, then naturally full-width images cannot be exactly the same when the one theme is narrower than the other: exactly the same would require inserting versions that are narrower than the width of the narrower theme. (I don't think that's what you're looking for, is it?)
I found a very big difference in Quintus between the large and the medium sizes
3. These options aren't immutable: you can, or rather you must, change them.
When you are about to insert an image, you get four size options: thumbnail, medium, large and full size. Full size means original width, if the original is narrower than the maximum the theme allows, or downsized to that maximum, if the original is wider. The defaults for the other three options are set by you in Settings > Media, and they should take into account the width of the theme: for example, the width for the large option could be set to around 3/4 of the main column and the width for the medium option to around 1/2 of the main column. As timethief showed, the width of the main column in Quintus is 750px, so I'd probably set the default widths like this: large around 650, medium around 400, thumbnail around 150 or 200 - or whatever looks better after I try it.