Movement & Space, 1st & 2nd Ed. Rules Question

There is a Bugbear occupying a ten foot square, (lets say four 5 foot squares).

Another Bugbear wants to enter the area, but the Rules Lawyer says it can't because the first Bugbear occupies the original ten foot square.

I get that a Bugbear is large, (7 feet tall). But I don't get that it should be 10 feet wide (or even larger than 5 feet wide), thus preventing the second Bugbear from entering the same ten foot square.

Does anyone know that rule?

Actually it's HackMaster, the parody of 1st Ed. The only thing I can find on the topic is number of figures (Medium versus Large) that can be around it.

Tracy Johnson
Old fashioned text games hosted below: