At least anecdotally, having moved here from Boston, no question that you're correct. You definitely get more space for your dollar in SF than in Boston. The reason is simple - they simply didn't build things as small in SF. People from the East Coast would probably be willing to live in smaller units for less...if they existed - but they really don't, at least not as part of the traditional housing inventory. (Some fancy new apartment buildings are experimenting with them, but needless to say, they're pricey, and at that price/size combination many would prefer to commute from Oakland.)
Even the concept of "Boston" is much smaller than the concept of "San Francisco". If you walk across Boston from any of its edges for an hour, you'll wind up in another town entirely (Allston? Cambridge? Maybe even Somerville). If you walk from an edge of San Francisco for an hour, you'll be smack in the middle of San Francisco, with a lot more walking to go.