Running into the same problem. When I try to place a thrall in a wide open space I get the “overlapping” error and can’t place them.
I also noticed I have destroyed random things like staircase, fence foundation, etc and thralls that are like 10 blocks away from what I destroyed will drop down a few inches like they were standing on something that was connected to this block when no such thing was true.
By “bigger” i assume you mean their hitbox is larger than the actual npc model. Sounds like a Bug alright. Thanks for reporting on the forum, Bug reports for Conan Exiles Enhanced should be sent here: Contact Us — Conan Exiles Help Center
I also noticed this bug during demolitions. When breaking building pieces very quickly, some non-targeted pieces were also demolished. It seems like the game can’t recalculate the stability of the pieces at the same speed as the player’s actions. I also noticed this issue in Dune Awakening and reported it to the team at the time.