Calling a Possible Semi-Bluff When the Pot is Large

First, you would call when the pot is large, even if there's a chance your opponent is semi-bluffing. Possessing any kind of competitive hand yourself, you certainly don't want to give away a big pot to a possible semi-bluff. So you can't fold. At the same time, there is no point in risking a raise since, because of the size of the pot, your opponent will call even if he is semi-bluffing. And if he's not semi-bluffing but has the best hand, he may reraise you. Therefore, the only play is to call.