The answer is in 314.28, which applies to not only boxes, but conduit bodies used as pull boxes.
6 times the size of the conduit for angle pulls.
The wording IMO is unclear, referring in A(1)and (2)only to "boxes" and then adding the conduit bodies in the "exception".
314.28(3) addresses the conditions under which a smaller conduit body can be used....it has to be marked with the maximum allowable number and size of conductors.(thus the 3-4/0 marking)