but will you get the body type of the one you disguise yourself as or just his clothing?
A fast heavy would be really suspicious
You look identical to the person you disguise as. So if you see a heavy climbing higher than he should, or a light who walks around the long way, you should be suspicious.
In the light disguising as a heavy scenario, i imagine they are able to mimic the moving speed.
If a heavy disguises as a light, though, it should be painfully obvious unless no one is paying attention.