Why are you betting pot here? Are you trying to make it look like a
bluff to get him to reraise you? If so, mission accomplished, jam your stack in. Are you trying to take the pot down right then? Betting small here accomplishes this just as well. You are either ahead or very far behind. He either missed this board entirely or has you crushed. If he has an over pair you are dominated. If he has a set or fullhouse you are dominated. The only thing you are ahead of is complete air or pocket deuces, fours, and fives. If you bet small here an he has nothing he is likely to fold anyway, but maybe he continues on with nothing and in the long run this adds to your profit. If you bet small and he has you crushed, he is going to reraise you regardless trying to get more money in the pot. Maybe he has AK and thinks you missed the board. By betting small you can call his reraise and reevaluate on the turn or you could even 4 bet here getting a better price because of your small bet size in the beginning and still fold to a jam.
Make sure you are understanding why you are betting. Bet to add money to a pot that you think you have the best chance of winning to charge your opponent to draw to a better hand. Bet to make better
hands fold when you don't think you can win the pot at showdown. I would not bet this flop with 77. I would check to him and see what he does. Maybe he bets, now I can check raise him getting his Ax and possibly middling overpairs that are actually ahead to fold.
I would highly recommend doing some study on bet sizing. There is a lot of content out there and I can tell you from experience that consistent, strategic sizing of your bets is half the damn battle of poker.