I thin you may be stuck if you cannot get them to agree since you offered to pay more for the larger room and agreed to take the apartment based on that. BUT I also think it sounds like they are taking advantage here (maybe they are smarting a bit because you got the larger room). I would tell them that you would pay $40 more a month but be willing to split the difference in the end and pay $50 if it comes to that since you should have made an arrangement about the rent before the three of you accepted the place.

If we go strictly by bedroom sq ft, your share s/b 42% of total rent and ea of your 2 roommates s/b 29% of total rent. Your bedroom is near 50% larger than the other 2 bedrooms

Here's your first life lesson as an adult...get negptiations and agreements in writing before moving ahead. Ie, you should have had figures agreed to in writing before you ever signed a lease and moved in.

If you can't come to an agreement, and aren't on the lease, I'd move out. Or offer to let one of the other roommates have it for 60. mo more.