I agree that it's not unreasonable to expect a discounted rent if the difference in room size is that great.
However..
The time to sort that out was really with your landlord and before you signed contracts.
The people you're moving in with are expecting to pay a certain rent, and if the landlord's not willing to budge post-contract signing, then the only way for you to get a cheaper rate is if your housemates agree to pay more than they were expecting to, which is actually rather unfair on them.
I would be annoyed if one of my housemates approached me just after signing a contract to ask if I could pay more rent so that theirs could be reduced because they didn't raise it beforehand.
I personally feel it would cause friendship issues, but that may be a risk you're willing to take and play it as diplomatically as you can.