Actually, the simple answer is 'yes' for this, and the proof for it comes from a long-standing bug in the game. Consider the various tunnel maps, where mobs are prevented by collision surfaces from getting all the way to where the floor of the tunnel meets the walls, because the walls slope inward. If a mob is knocked down, though, their collision surface orientation changes to horizontal, reducing its height, allowing the mob to be knocked 'under' the angled wall. When the mob stands up, the game doesn't check for collisions, allowing them to stand up and putting the center of their collision box on the wrong side of the wall collision surface, causing the wall to obstruct line-of-fire with single-target attacks -- the 'untargetable mob stuck in the wall' bug.
The answer to the question you quoted is actually 'no'. The player collision capsule is a fixed size. It's equivalent to a cylinder 3 units tall with a 1.5 unit radius, capped by two 1.5 unit radius half-spheres, one on each end. Imagine an upright pill capsule sitting inside a 3x6x3 box. It does
not scale with the costume size. This can be easily verified by making extremely large or small costumes with database editing and noticing that it does not affect player collision at all.
Other entities default to that same size, but can be overridden in the entity type data file. It's a fixed size for that entity type regardless of how many different costumes it can wear. Most larger NPCs have a (manually) increased collision capsule size. For example, Nemesis Warhulks are set to 7x10x7, which is closer to an elongated sphere than it is to a cylinder. There are a couple of critters that have special handling for collision so that the capsules are attached to their bones instead of using a single one, but I can count those on one hand. The Avatar of Hamidon is one such example. Again, the collision is fixed and cannot change for an already spawned NPC, or even for different NPCs of that same type.
The issue with knockback you're referring to has nothing to do with normal collision at all -- entities are never actually pitched when being knocked back. Entities normally have 0 pitch except under very rare circumstances -- when flying mostly, and usually then only under player control. They just have an animation that makes it
look like they're on their back, even if the pitch is still 0. Also easy to confirm in Paragon Chat using the collapse and swoon emotes.
The bug you're talking about has to do with ragdoll. When a critter (NOT a player!) is knocked back, if the magnitude is sufficient, the entity goes into ragdoll. At that point, normal collision is switched off and everything is handed over to the PhysX library*. PhysX uses the actual bones of the model and collides them with world geometry -- this is how an arm can catch on a handrail while the unfortunate critter tumbles over for instance. While in ragdoll, they can squeeze into smaller spaces due to the actual skeleton being used. Once ragdoll ends, the normal simplified capsule-based collision model kicks back in and the entity is stuck in the low-ceilinged wall they slid under.
Nonstandard code rant over.
* PhysX is used clientside for non-shared objects like debris, but serverside for ragdoll so that everyone saw the enemies in the same state. That they were using the same library on the server is speculation, but is a good bet because it would mean less developer effort as the same code for interfacing with it could be reused, and I'm pretty sure I remember developer comments about the servers having Aegia cards. In any event, a reimplementation would not need to use the same exact method for ragdoll, if it was even supported.
