[gisb0rne]

In my experiences bigger bets on the river aren't usually bluffs. I've called plenty of big river bets in sequences just like this and most of the time ended up facing the flush/straight or something else that beats me.

Here's a reasonable thought process for him if he has a Q: "I caught a Q on the turn but the board is kind of scary so I'll just bet the minimum. Ok I got a caller but he didn't raise so my hand is probably best. The river was a dud since he likely doesn't have a ten, so I'll make a big bet to get the most out of my big hand."

Here's a reasonable thought process for him if he has a flush/straight: "I've got them beat so I want to entice them to donate more money. Thus I'll make a small bet. Ah ha! I got a caller. He probably has a queen so he'll call a bigger bet on the river. Here goes..."

I'm not suggesting that calling is wrong. It's just that I can easily see him as having a better hand than you and still doing this betting pattern.