Also need to point out, Large monsters come in smaller groups, meaning the possibility of the heroes focusing fire on one is increased, as opposed to dealing with a swarm of smaller opponents.
With a 4 hero group, that means 2 large monsters taking all of the attacks and you only need one really lucky shot to down a monster. The OL for our group was quite flustered when our healer, Elder Mok, turned out to be able to oneshot his Arachyuras. Depending on the group composition, and armament, a large monster groups could conceivably be gone after the second of the hero's character's turn, leaving the other heroes to go after another monster group or a quest objective.
Large monsters have their qualities, but quantity is one of the qualities they do not have and when it's been demonstrated that it only takes one hero to down one of the OL's large monsters, the ability to reinforce only one small or one large monster per turn becomes moot.