I don't recall this being an issue before, but then I haven't really extensively used DOSBOX for running Windows before... however there seems to be an issue (0.74) that if you set the processor to Pentium_Slow then the Expanded and Extended memory default to 1MB each.

Using Auto, or in my preferable case 486_Slow this issue disappears.
Now as far as running Windows 3.1x or NT 3.51 is concerned this isn't a big deal as really they tend to run better under 386/486 environments, but it probably would be something to note given certain titles (such-as Terminal Velocity) have specific Pentium paths that enhance the features rather than just performance.

As I said, not a big deal but might be something to check out.
Also as I've notice it seems damn near impossible to get ahold of, actually copied the original S3 Trio 32/64 V2 Drivers for Windows 3.1 from a floppy drive earlier today.

I will note though the 1Mb version DOES NOT WORK with DosBox, you get corrupted graphics / blank screen. The rest work just fine though.
Kinda funny to think about but when I found the disk wasn't until I went to put it in the disk drive that it suddenly dawned on my system hasn't had an FDD in god knows how long 😄 so had to actually hunt one down from the attic.

Be sure to mount it to A:\ though, for some reason trying to do it from mounted C:\ tends to cause Windows to complain there's no oemsetup.inf - probably some legacy reason behind that but god knows what.