Ncdmt, think about the opportunity cost to see why your analysis is off. Op could have the smaller room for 400 dollars, but instead has the larger room for 550 dollars. Assuming all else is equal (he isn't entitled to any other benefits), this implies that op is paying $150 for the larger room.
To the op, whether you got shafted or not depends on how much bigger the room is. Since I gather it doesn't have a private bathroom, it seems likely to me you came out on the short end here. I'd renegotiate. Just say you had offered to pay $75 more, and that you aren't sure how you ended up on the hook for paying $150 more. Tell your roommate that you'd like to enforce the original agreement going forward. While it isn't a lot of money per month, this adds up to the better part of a grand per yr, which is a mediocre laptop/small TV/two month's rent...If your roommate fights you on this, offer to switch rooms (don't declare war/stop paying rent--this will make the rest of your yr pretty shitty). If your roommate complains about moving, offer to hire movers (assuming $75 is the "fair" price, you are losing $75 a month, or $900 a year; you can probably get a guy or two to swap your bedroom furniture for $100 bucks, saving you $800 on the year (call your $100 loss a learning experience). That's how I'd play it anyway.
ETA: The above isn't guaranteed to work. If your roommate decided to stay in the apartment/signed the apartment in the first place largely because he could only afford $400/mo or only wanted to live in such a place for $400/mo, you're boned.