[wmf]

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.

[silentbicycle]

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.)