[Solitare]

While betting the flop is fine, I'm not a big fan of the size of the bet. To me it says "I've got top pair, but I'm afraid of draws, so I'm going to bet more than the pot to let you know I have top pair so you will fold."

I think this is a case of strong looking weak. I think the size of the bet encourages the big stack to try to buy the pot with a semibluff, even if all he has is two overcards and a backdoor flush.

I'd bet out t300-350.

I would have also raised pre-flop. This sort of flop is exactly why the pre-flop raise is important. If Villain is semibluffing, your pre-flop weakness might have been a factor in Villain's decision to make that move. If you had raised pre-flop, Villain would have to be concerned that you had a pocket pair. Or an A5 giving you a gutshot straight draw to go with your pair. These possibilities could discourage a semibluff.