Weighty Second Acts

From what I've seen, most musicals seem to be built into their First
Acts more than their Second. That is to say, that the Second Act seems
to tag along as just the conclusion to the First Act. Certain musicals
handle this better than others. Wicked's second act holds because it
ties the story into the Wizard of Oz, which is what sold the story,
while one reviewer said that Avenue Q had little to offer after
intermission. In My Fair Lady, Eliza's arc is already complete by the
end of Act I. Meanwhile, at a certain point, South Pacific stops its
numbers in ACT II so that the storyline can move forward, with Emile
and Lt. Cable spying on the other island.

So, I'm interested in studying musicals where the Second Act upstages
the First. Has anybody done it before? How successful has that been?
How climactic is the resulting dramatic structure? Questions like that.