Most manufacturers use only a few different size chassis for their receivers. The usual brands (Denon, Yammy, NAD, HK, et al) to the best of my knowledge do not have a smaller chassis size. Doesn't mean they dont have one, just I am not familiar with the smaller chassis.
I have seen small chassis from some "artsy" companies (Bang & Olfusen, Bose come to mind), but even their receivers then to have some moderated depth to them, and then to be thiner, not shallower.
There may be some low priced entry level equipment from Panasonic, Technics, even Radio Shack, that may have a smaller chassis size.
Either way, remember, you will have cables etc at the rear that will require additonal depth, and the modern receiver is going to get warm, which may be a problem with the built-in.
