It's the rigidity of form, particularly the 32-bar song form, that helps convey power... It is for me much more effective, though, if you can tell a story through 32-bar songs – or even 33-bar songs, but not through 133-bar songs.
Many people who are familiar with Stephen Sondheim's songwriting might find that statement as weird as I do: Sondheim is perhaps the most long-winded great songwriter in the history of musicals, and his biggest fault (in my opinion) has been a tendency to write songs that are too long. But even if you don't agree with me that "A Little Priest" from Sweeney Todd is twice as long as it needs to be to convey its one joke, or that some of his other comedy songs are coma-inducingly long, to think that the statement either shows a) A lack of self-awareness or b) A sense of self-criticism, like maybe he thinks his songs sometimes got too long. Or maybe his point is merely that today's musicals are long-winded even compared to his work, which may or may not be true.
One reason why Follies is my favorite Sondheim score is that the need to write songs that were homages to old Broadway show tunes seems to have had the effect of disciplining him -- either that or the fact that it was in part a dance show, and some of the songs had to be short enough to incorporate substantial dance material. Whatever the reason, even the numbers that aren't pastiches are pretty normal-length and stop when they're finished saying whatever they're saying. But when he returned to the property and wrote new songs for a 1987 revival, they were... incredibly long and with a tendency to re-state the same ideas several times (plus a tendency to keep going off in new musical directions instead of coming back to the A section where they started). But many other Sondheim songs have the effect of slowing down the show by their refusal to hurry up and say what they're trying to say. His own favorite song, "Someone In a Tree," lasts seven minutes, and while it is a very fine song, it's... really, really long, and has two refrains whose structure is so convoluted that it takes you two listenings to even identify them as separate refrains. And by Sondheim's own definition, it might be more theatrically effective if it weren't so long.
This is one reason why the Sondheim songs that hold up the best are often the ones that are -- not necessarily short, but economical. (Conrad L. Osborne once wrote that Wagner's Gotterdammerung is "an economical score" despite its length, because everything in it is essential to the story. And to use a Broadway example, "Soliloquy" from Carousel is really long because the character goes through several huge mood changes within the course of the number.) The Act I finale of A Little Night Music, "A Weekend In the Country," is my favorite Sondheim song, and while it's long, it's long because it has a lot of story and action to put across (it was written, it was said, after Hal Prince had already blocked all the action); structurally, it's a series of traditional-length AABA refrains.
And sometimes it's just better to be short. "Finishing The Hat" works better than most of the songs in Sunday In the Park With George (a show I'm not fond of, admittedly) because it's short, makes its points concisely in a verse and two refrains of normal length. And let's finish this off with Dorothy Collins singing "Losing My Mind" from Follies. It's a short song because it's a pastiche of Gershwin-style songs, just one AABA refrain which is then repeated verbatim (not even new lyrics for the second refrain), but would it really tell us anything more if it were replaced by a seven-minute musical monologue?