I believe I've mentioned this before, but anyway, the hitboxes in DT are almost always smaller than the sprite.
When DT1 first started, there was none of this. Every sprite was its own collision, which led to a lot of problems.
For some enemy sprites that have a weapon as part of the sprite itself, I'll often cut off the weapon for collision checking, both due to it being awkward for the player when they take damage from it (I think so anyway) and that it's also part of the collision for the player to deal damage to the enemy. For those occasions, a separate object is used (as you would imagine) for the melee weapon the enemy has.
The above collision examples are only for collision with damage objects, these are not used for world collision. (though they can be) That's handled in a different, slightly more complex, way.
These are in DT3 as of a moment ago. There's been plenty of talk with people about these and if they should be in the game. I've shared my thoughts on them before, and I tend to not like them. So, nearly all of the achievements in DT3 are very silly things that are outside of normal gameplay. There won't be any "Completed Chapter 1" or "Defeated 100000 Enemies" achievements. Most of them are designed for comedy purposes.
A comment here inspired someone to say a thing to me, which inspired me to code in achievements. Luckily some of the UI and code was already done from when I was toying around with the idea.