The language-lawyer tag means that you are asking about C++ and its compliant implementations, not some specific physical machine.
I'd have to quote the entire standard in order to prove it, but the simple fact is that it makes no guarantees on the result of sizeof(T*) for any T, and (as a corollary) no guarantees that sizeof(T1*) == sizeof(T2*) for any T1 and T2).
