Week preflop raise

I was in a mtt the other night, when everyone folded to me and there were 3 players after me. (i was the big blind) i was sitting with AJ off, so i put in a minimum raise of 500 had about 4000 chips. others had around 6/10,000 chips. two folded and the last one called, the flop came A37, so then i went all in and the guy called me , He had A3, i was out. i thought he was a monkey for calling my raise with A3, but soon realised he probably would have folded, if i had raised alot higher than 500. He must have thought that if i had a picture card ace i would have raised higher , so he called for 500 thinking it was a bargain . oh well you live and learn. (ps the blinds were about 150- 300.)