Force. Specially, Intramuscular force. For some amount of volume.
What seems to lead to a large amount of intramuscular force production? High threshold motor recruitment (ideally from the largest percentage of individual contractile fibers). Which seems to be caused by: load (high percentage of one rep max), unintentionally slow rep speed. Which is why training in close proximity to failure is important (both these occur there)
Everything else…
Chris Beardsley – Effective Rep Theory