I sympathize with you, but sort of the opposite way. In college, I lived with four roommates in an apartment that had five rooms of greatly varying size and amenities. We decided that the simplest thing to do was to all pay the same rent and pick straws as to who got which room. Guess who got the smallest room with the smallest window and the best earshot of the kitchen? But at least I agreed to the plan before I was stuck with my lot.

The "steepness" of the $60 depends on how much the total rent is (i.e. what percentage more do they want you to pay?). The reasonableness of your wanting it to be $40 also depends on that. But regardless, things would be easier for you if the actual amount had been decided on when you agreed to pay a little extra for the larger room. You'll have a good reason for insisting upon a clear agreement with your next roommates :)

But as it is now, you have to come to some agreement with them. I don't think you want to fight about this for very long. So I would recommend to come to some sort of agreement and then whatever it, just let it go (avoid stewing :). If you're stuck paying the $60 instead of $40, it's still only $20 a month or $240 for the duration of the lease. Even if you are a poor student (I've been there!!), that amount of money is not enough to dwell on for too long. Although you didn't come to an agreement beforehand (which is what helped me to just "live with it"), I think you need to at least talk to them in order to avoid dwelling, even if nothing changes. As for coming to whatever agreement:

--Are there other things besides the size of the room that make it more desirable than the two smaller rooms, or vice versa? (for example, some people might say that a room with a balcony or porch is worth more than a larger room that lacks a balcony. Another example might be closet space.)
--Would your roommates be willing to switch with you, and pay the same amount of money? You could volunteer to switch with one of them if you feel very strongly about it. Hopefully, this will put your roommates in your shoes in terms of considering the worth of the bigger room. Of course, they may really think it's worth it, or that it's even a bargain, and take you up on it, so I wouldn't make this offer without serious intentions behind it. (Whether or not you'd be willing to give up the room for the $60 may also give you a better sense of whether that $60 is worth it or not).
I guess you could always say that you'll pay the extra money, minus a usage fee for the couch, tv and kitchenware, but that's not really going to be a positive impact on your friendship. I'd avoid it, if these really are your close friends. You seem to have brought those things to the apartment as a gesture of goodwill and generosity. If everything is really so friendly between all of you, you can expect they will at least discuss the issue of the extra money with you.

Hope this helps --

This is something you should have hassled out BEFORE the lease was signed.

OK: you have a bedroom that is 150% of the size of theirs ... but that's not all there is to an apartment. What is the square footage of the apartment? And what is the rent?

I would tell the others that while you do agree that you offered to pay a larger share you think that your share should really be $XX since you did contribute more of the kitchen items and living room furniture.

If they have a problem with that ask them to compromise and split the difference between the $60 they want you to pay and the $40 you think you should pay. If they still have a problem think about getting new roommates. Or just suck it up and pay the $60 until the year is up and then get new roommates.

I've had numerous roommates and I can tell you for a fact the worst feelings/conflicts were over things we DIDN'T openly discuss. Things you let bother you, or they let things bother them and it blows up. So you need to work it out, or decide it's just one of those things.

And also sit down and really get a handle on who is going to pay what for utilities, cleaning supplies, cable, etc. And who will do work around the house.

BTW I do remember a time, not so long ago, when $20 was a huge deal to me. I worked in retail and was in grad school and had 3 roommates at a time. I have SO many roommate horror stories and they all began with me not speaking up when I saw something that was wrong or bad or unfair

If you don't want to pay the 60 , ask one of them if they would take the bigger bedroom for the extra 60$...
If the answers no, put the room up to bid....
(when much younger , we did this once, on paper write how much more you are willing to pay, then open it... if there's a tie, the two who tied then bid again.
hint: on the first round all increments must be in 10s, in the second, $5s.....)
ask here if i'm not being clear.

what else you are bringing to the apartment has no bearing (cause you would have brought it anyway, right?)

The room is only 1' by 3.5' bigger? Does it have its own bathroom? A much larger closet? Or anything that makes that little bit of size difference worth $60.00 more per month?

Unless you can get one of them to trade rooms with you or figure out where they came up with the number of $60.00 and argue it, it sounds to me like you are stuck until the lease is up IF your name on the lease?

You didn't like the apartment but said. "Ok, fine, let's take the apartment if you don't mind me taking the larger bedroom, I can even pay a little more for it if you'd like."

Kitchen stuff and furniture, I am guessing you offered that too.

Never, EVER offer to pay more or supply more in a room mate situation until asked.