Fair comment, it's been many years since I watched the REM live video with a bunch of extras. I was watching a lot of U2 at the time and I remember thinking it was more impressive them doing what they do without the extras. Queen also produce a massive sound without help. So yes, FM should definitely strip it back some and I'll withold judgement on REM until I've researched Tourfilm or whichever it was in 1997ish....
Tourfilm (‘89) definitely had one extra player on stage (Peter Holsapple), I don’t remember how heavily he features in the film but he was nearly made a full member around that time, so it’s possible he’s in it quite a bit. Roadmovie (‘95) had both Scott McCaughey and Nathan something, which I’d bet is the one you’re thinking of.
The other two groups you mention are really interesting choices. U2 has never had a fifth member on stage, true, but from ‘84-present they’ve used backing tapes to fill in the gaps - sporadically from ‘84-‘90, but from ‘91-present the vast majority of their sets have pads underneath, all masterfully controlled under the stage by Terry Lawless. On the 360 tour they would occasionally put a camera down there and show him on stage.
Queen from ‘73-‘81 were just the four of them, and you’re right, they made a HUGE sound. In ‘82-86 they had a keyboard player offstage (Fred Mandel in ‘82, then Spike Edney from ‘84-‘86, Spike has been Brian’s keyboard player ever since).