To explain why you're being downvoted into oblivion:
ints remain the same size, but longs and pointers are double the size. 64-bit libraries may be bigger, but there's no reason they would double in size. In general, a 64-bit system uses more RAM, but not double.
One data point: IIRC, a benchmark I ran comparing memory consumption for Lua coroutines (co-operative threads) used roughly 1.3-1.4x as much memory on 64-bit as 32-bit.
I'm not sure if 1.25-1.5x would be accurate as a rule of thumb, but 2x is almost certainly too high. (It will depend on the data structures etc. involved.)
ints remain the same size, but longs and pointers are double the size. 64-bit libraries may be bigger, but there's no reason they would double in size. In general, a 64-bit system uses more RAM, but not double.