I met Christian McBride briefly in college a couple of times. He is certainly a powerfully built guy, but not that big. I seem to recall him being slightly shorter than me, so around 6ft. I think if you put him under a shrink ray and made him 5ft, he would still find a way to pull a big sound without over exerting himself.
I also met Linda Oh, and she performed on my (well, the school's) bass. She was only in her mid 20s at the time, and despite being a small fraction of Christian's size, she got a really great sound. Maybe not Christian McBride big, but I attribute that mostly to personal taste/style, experience, etc. It was a nice, big, fat sound as best as I can remember; one that I think Ray Brown himself would be proud of.
Another player who comes to mind--but is a bit off topic since he's a classical player, sorry--is James VanDemark. He's a small guy and gets a f---ing huge sound of the bass, anyone's bass for that matter.
So suffice it to say that I don't think physical size or strength is all that important. With proper technique, it really shouldn't take that much force or energy (I mean that literally, in physics terms) to get a nice big sound. Now, I do take issue with the "petting the cat," barely caressing the strings approach, but you sure as heck shouldn't be wrecking your hands trying to get a good sound. Just my 2 cents.