I poured through the FBVector code because of your comment. It has some really interesting tricks. In a normal array allocation, it always goes for the next-largest-size in the jemalloc memory hierarchy. That is, it goes for multiples of 64 bytes, 256 bytes, 4 KB, or 4 MB. This makes the array cache-efficient.

The push_back() semantics features the standard array doubling technique, but only up to 4 KB; jemalloc can't grow in-place anything smaller than this, so a copy is required. Beyond that cut-off, push_back() will instead grow by 1.5 times the capacity to prevent too much "slack" memory from accumulating.