The B-100 has *much* beefier caps after the rectifier, and a bit more filtering after that. These are both common (especially the first part) in SS amps vs tube amps. While electrolytics can be smaller today than in the 60s, they're still not that small in terms of space in an amp. A modern 47uF 450V cap (the largest typically used in most tube amps) is a bit larger than my index finger from its tip to halfway to the second knuckle. A 4700uF like in your B-100 would be far larger, take a while to charge, and require the use of a SS rectifier rather than a tube rectifier.
Then, as I said, there's even more filtering going to the other stages before the power amp. Whiel tube amps typically have this, too, the caps are again larger in this amp than in a tube amp.
There may be more to it, but that's a big factor.
