Oh HELL!!! depends on your YEAR of reference... 100 years ago, a 577 BPE was a medium, a 40 was a pistol, and a 4 bore was a heavy...
Here's THE drill, and i disagree with all my brothers on this one.. if it's legal to hunt Big Five, it's a big bore... 9,3x62 is FAR more of a big bore than a 50 AE, even though we are talking .366 vs .500 (yep, .500) bore.
IMNSHO 375 IS a light big bore, TODAY, all 40+ rifles are, and anything pasy 458 aint a question...
<whew> 500 jeffe is the KING of big bores... hey, did I say that out loud?