I think all of us--or at least those of us who've both owned and handled the BSS doubles, both boxlocks and sidelocks--will agree that they're very different critters. Depending on barrel length, a standard grade boxlock (not the straight grip Sporter, which is lighter) can push 8 pounds. I doubt a sidelock 12 would ever tip the scale at 7, although I might be wrong on that score.
I know one of the posters that talked about the clubby feel of the BSS Sidelock pretty well. He spends a lot of time hunting pheasants with a Prussian Daly that's in the sub-6# category. (I owned one of those at one time--a Featherweight--and it was the lightest 12ga double I'd ever handled other than Brit 2" guns.) But the thing is, you're not very likely to find a sidelock--British, Spanish, Italian take your pick--that will fall into that category. Lightest one I ever owned was a Charles Lancaster at about 6 1/4 with 28" barrel and fairly thin (but safe) wall thickness.
But weight is only one of the factors that can give a gun a "clubby" feel. Another is the profile of the grip. Thin-gripped guns with splinter FE's may not weigh that much less than a gun with a thicker wrist and a bit more wood under the barrels, but they can feel that way. Then there's the issue of balance. Ruger did a decent job of shaving weight from their sxs Gold Label, but the result was a gun that's nose-heavy. Most light game guns, I think, have a more neutral balance, or perhaps slightly barrel light.
And how all of that feels depends to a great extent on the preferences of the individual.
