https://github.com/facebook/folly/blob/master/folly/docs/FBVector.md здесь написано, что выделять память для вектора по степени двойки - плохо. Но вот причину я не могу понять:
What that really means is that the new request for a chunk will be never satisfiable by coalescing all previously-used chunks. We would of course want the vector to not crawl forward in memory, but instead to move back to its previously-allocated chunks. Any number smaller than 2 guarantees that you'll be able at some point to reuse the previous chunks
