Size or numbers do not matter.
(That's what she said)
As seen in the Battle of Thermopylae.
No seriously, Overloads and Behemoths are nothing but bullet sponges, but if they were introduced to the West Coast Damage Threshold mechanics, I say they would fail horribly. No contest, yes. :fallout:
Notice how much tougher Giant Radscorpions got from Fallout 3 to New Vegas?
If Overlords and Behemoths were in NV, they would both have either absolutely
massive HP pools, amazing DT, or both, as they are very high level enemies and as such would be statted appropriately.
Numbers also most certainly did matter in Thermopylae; the 6,000 or so Greeks there were overwhelmed by an absolutely massive Persian army over the course of three days. Much longer than it would have taken if they fought in the open, certainly, but numbers most definitely decided that battle.
Overlords and Behemoths are also not just "bullet sponges"; they are among the highest-damaging enemies in the game, as well. Behemoths hit like trucks, and Overlords typically carry the extremely lethal Tri-Beam Laser Rifle. In Broken Steel, an Overlord can mow you down in nothing flat with one of those things.