This is not only cleaner but also helps make sure we are always using
the virtual size measure that includes the sigop weight heuristic (which
is the vsize the mempool would return).
This is a b)rtial ackport of core#23381
https://github.com/bitcoin/bitcoin/pull/23381/commits/36a8441912bf84b4da9c74826dcd42533d8abaaa