That's larger, not more. Much of the argument was allowing extra bags for first class passengers. I don't know of too many places that allow even first-class passengers to bring on (or check) larger than normal bags, just usually allow one or two free bags instead of paying for them.By carrying one 33 inch bag rather than two 28 inch bags.Then I'll ask: how do you board with more than the allotted carry-on luggage amount without creating a safety or delay issue?
If sleepers truly allow for larger bags than coach does, then yes, the policy should be amended. There may be room in a bedroom for a larger bag against the wall, especially if you're the only one in it. However, it'd have to be able to be out of the way for liability reasons (for example, if there's an evacuation needed there can't be luggage blocking the door or pathway to get out of your bedroom.) A roomette doesn't really have any more room than a coach seat for storage, and coach and roomettes both have the same luggage rack downstairs on Superliners.
I really can't imagine that coach requires smaller bags than sleepers do, though. I trust (perhaps naively) that Amtrak has set the carry-on policy at an amount that can fit in the proper storage areas, and it's set at the limit of what that can be. Proof otherwise would be nice, especially if that proof shows that coach can only handle the current baggage size, but rooms can hold more safely.