Okay, so the tiers are for the hitboxs and not the collision boxes.
While I was working on various oki setups for W.Len using super jump to push people into my 421b, i noticed that some characters (Lolis, Akiha, and Ciel) have smaller collision boxes than the rest of the E-tiers (Sion, Ries, etc...). I don't have any real exact numbers so if you can look into that, that'd be fantastic.