What is the reason that B21 is being considered smaller than the B2?
If the engine is based off F135 like F118 off the F110, then dry thrust is likely to be close to 15 tons *2 which would put the power on tap equal to the B2.
It depends specifically on how much thrust the F135 variant for B-21 has.
F118-100 for B-2 had 19,000 lbf each (
), about 14.4% more thrust than what F110 had for dry thrust.
Four F118-100s provide (19,000 x 4) = 76,000 lbf of thrust total.
F-135's dry thrust is 28,000 lbf of dry thrust. Assuming they uprate it for dry thrust by 14.4% as well (for the sake of discussion) you get 32,032 lbf of thrust.
B-21 by our understanding is meant to be powered by two of these F135 dry thrust engines, giving us some 64,000 lbf of thrust total.
As far as the assumption that B-21 will be a little bit smaller than B-2 goes, the slightly lower total thrust that the B-21 will likely have at its disposal vs what B-2 has is definitely a factor.
The USAF's public announcements of what the LRS-B (which became B-21) and the consistent noises made about how it would be slightly smaller than B-2, are also a factor as well.