Most of what either woman says in “The Roommate,” which opened Thursday at the Booth Theater, is greeted by one or the other response. The two actors, old friends and old hands, play beautifully off each other, expertly riding the seesaw of a play, by Jen Silverman, that throws a Bronx grifter looking to reform herself into an unlikely alliance with a flyover frump looking to ditch her flannel ways. The actors’ intense focus and extreme contrast multiply the material exponentially, sending it way past the footlights to the back of the Booth. But as we’ve learned, sustaining and expanding are two different activities. Indeed, the Broadway supersizing of “The Roommate,” which has been produced regionally since 2015, does not necessarily represent progress, even as it no doubt reaps profit.