Obviously the pointers take twice as much space but I don't think that is what you are asking.

arc runs in MzScheme 352 (http://download.plt-scheme.org/mzscheme/v352.html)

the freeBSD binaries on this page are i386, if thats what the new server is using then it won't make a difference. The source is available as well, if pg compiled the source in 64 bit mode it would increase memory usage.

But using a 64 bit OS doesn't necessitate using a 64 bit address space.