Going by the size of the B-2s weapons bays then the bay on the B-21 should be the same size, Northrop would not design the B-21 with a smaller bay than the B-2 even when it is only one considering the size of the GBU-57. The USAF would not have given Northrop the contract otherwise.
In fact if anything, the B-21's bay seems slightly longer, which might be fed by the requirement to carry LRSO. There is no doubt that the bay has sufficient volume; the only question is payload. I find it hard to believe that the B-21 cannot carry 30,000lbs. If we assume LRSO is in the same size and weight class as AGM-86, that is already 25,000lbs plus the weight of the rotary. It is hard to imagine 30,000 would be difficult.