std::vector capacity after copying

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All you’re guaranteed is that:

  1. The vector has enough capacity to store its elements. (Obviously.)
  2. The vector won’t get a new capacity until it’s current capacity is full.*

So how much extra or little an implementation wants to put is up to the implementation. I think most will make capacity match size, when copying, but it cannot lower capacity. (Because of number 2 above; reallocating while there’s enough room is not allowed.)

* Mostly. See Charles’ comments below.

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