The heavy infantry's shields are a good deal larger than those carried by the medium infantry, which further highlights the contrast in model size. In my view the medium infantry's shields could stand to be a bit larger, and the heavy's shields should defintitely be smaller.
The scale difference won't stop me from using them together. There are a few tricks to making the heavies look taller, and I might try trimming their shields. Still, it's a shame that all of Hat's Late Roman offerings weren't sculpted with the same proportions.