By one interpretation you could just say base Saiyans just became that much stronger.
Or you could say that Trunks' reaction isn't that meaningful since he never actually puts a quantifiable value on Piccolo's growth, just a generalized statement.
From the way Future Trunks reacted, it makes me think that Piccolo had a bigger power boost than Future Trunks in the Rosat.
I think it's a very good solution to have Adult Saiyans in Arc Boo with a small SSJ multiplier. In addition to having: Base Adult Saiyans > Piccolo > Future Trunks ASSJ, we can also make sense of things like:
1.-) Base Gohan not being much inferior to Goten SSJ.
2.-) Base Gohan (M10) stronger than the SSJ Children.
3.-) Goku Base not being astronomically inferior to Majin Vegeta SSJ.
4.-) Base Gohan (Post Espada Z) being perhaps > Gohan SSJ (Pre Espada Z).
In BoG Goku and Vegeta are tremendously stronger than in Arc Boo, I would say that the SSJ multiplier was once again 50x and of course they also became stronger in all their forms.