Investigate virtual memory growth 2.2->2.3

With some data in db directory:
2.2 mongod at startup: ~300mb of virtual address space
2.3.2 mongod at startup: ~1237mb of virtual address space

With empty db directory:
2.2: 126mb
2.3.2: 433mb

I tried removing FTS and rebuilding, but that only reduced the amount by about 10 mb.