performance comparing ubuntu 12.04 32-bit and xubuntu 14.04.1 64-bit

we all knew xubuntu is lighter.
so i expected my program ran faster...

I had my cpp project compiled using g++.
the result, average of 10 tries, on a quad core computer:

ubuntu 12.04:       4.89 sec
xubuntu 14.04:     5.12 sec

unexpectedly slower in xubuntu, with 2 years more advanced.
i guess, it consumes less resources, so less optimization and preloading/caching, so slower (?)
or bigger foot prints in some latest libraries.
or 64-bit has bigger overhead?
or more recent libraries are bigger in size, more checkings?
anyways...