The thumbnails Galerie generates for video clips (using the default Quicktime) are much larger than the thumbnails generated for photos. For example, I have the max thumbnail size set to 125. My photos and video clips are both 16:9. The photo thumbnails come out 125 x 93, but the video thumbnails are 222 x 124. (The video thumbnails are composed of 4 smaller images from the clip.)
Is there any way to change this behavior, so that all the thumbnails are the same size?
I have never seen that behaviour. Thumbnails (the maximum size of the long sides) were always the same size. The video thumbnails are indeed composed of 4 frames out of the video. I do not know if the proportions play a role. I never used 16:9 video clips with Galerie and I doubt that Galerie was ever tested with 16:9 clips at the time it was developed.
Have you tried different sizes and/or different templates?
Can you show us an example (url of an example gallery)?
Apparently, the setting "Apply maximum size as maximum height" in the Pictures section affects movie thumbnails. This is no doubt a bug in Galerie. This bug was not discovered or reported before. Galerie is no longer in development, so there will not be a fix unless a new developer wants to continue the project.
What you can do is generate two galleries, one with and one without that setting, and replace the index files and "thumbnails" folder in the first gallerie by the index files and "thumbnails" folder from the second gallerie.
(Note that you will probably need to empty the cache of your browser to see the changes.)