HP Scanjet 5370C stopped working correctly after FreeBSD 'pkg upgrade' to
sane-backends-1.0.31_1 (FreeBSD 12.1-RELEASE p10). The image of scanned 8.5x11 inch letter size document ends up 1/4 the size of the original. The problem is nonexistent for my backup machine (FreeBSD 12.1-RELEASE p6) using sane-backends-1.0.29. Both machines are using xsane-0.999_6 for a frontend.
Just for grins I loaded up Linux (Void) on a test box and installed sane-1.0.31_1. Testing again with an HP Scanjet 5370C gave the same quirky 1/4 sized scan. No amount of setting changes in Xsane affect the image scale, in either FreeBSD or Linux.
It looks like a problema upstream then?
Try to report upstream.