It does indeed appear to be a bit on the big side - 1/35, I guess? I had one converted as a Chaos tank many years back and suffered from the same problem.
I wonder if you could add a suitable tank commander (and maybe a driver) to give it some more scale. The "problem", if you want to call it like that, would possibly be that the M113 is so instantly recognizable.
IIRC, Ironworker did a nice job of "downscaling" a Piranha toy by adding anti-gravity banks in place of the wheels.
Paintjob is nice and dusty, though, and it would be a shame to spoil that.