I'd guess that most of the memory is used by blocks of text which are much larger than the pointers which reference them, so probably the size_t expansion has a minimal impact on the total memory usage.
That's true, but 64 bit will affect other things as well - pointer size goes up, your stack will be bigger and the packing (alignment) of your structures changes.
If your compiler is unfriendly, your structure size can change significantly -- afaik, GCC is pretty clever about structure packing, but I'd imagine it varies a lot depending on the architecture.
