ALL the Borg ships encountered after BOBW were smaller than the one from Q Who?/BOBW (I stand that they were the same ship). The one in FC was smaller, and VOY kept to the standard set in FC (because that one was the CGI model in the system).
The Borg couldn't destroy the ENT-D in TNG either, that massive ship somehow couldn't destroy the Ent-D in a single shot the way it did to the others at Wolf 359.
And there weren't any other Galaxy-Class ships at Wolf 359, they were mostly older vessels like Oberths, Mirandas and Excelsiors. We saw a Nebula and an Ambassador but no Galaxy-Class.
The Intrepid-Class was a post-Borg ship like the Defiant, so naturally it would have more survivability against the Borg.
I suppose if Voyager ever used a solar flare to destroy a Borg Cube, there'd be an outcry of "Voyager shouldn't be able to do that!
" or "The Borg are more powerful than a star, its' power shouldn't have been able to SCRATCH them!