Just thought of another movie -- Pain and Gain. Mark Wahlberg is Tom Cruise's height, Dwayne Johnson is 6-5. Not a big deal there either.
I don't remember saying it was a big deal. It just struck me as an amusing juxtaposition is all.
What also made it stand out to me is that unlike the other examples mentioned, Cruise is an A-list megastar; Walhberg isn't really in the same league. As for the Avengers, well, that was an emsemble group piece, with our heroes frequently off doing different things in different places, while TMFU will have two leads, presumably sharing a lot of time onscreen together. The contrast will be quite apparent.
But I guess it's to Tom's credit - quite a few egotastic movie stars would have nixed being cast opposite someone who will make them look tichy onscreen, particularly where Cruise is the big name and Hammer the up-and-comer with considerably less clout and box office appeal (as yet). As with Mission: Impossible Ghost Protocol, where the studio insisted that Jeremy Renner be included as a co-lead/possible replacement, Cruise has shown that he is quite content to put the good of the movie first as oppose to his ego.