We cannot forgot he wrote the vocal melodies for basically what sounds like all songs he did lyrics for. So that gives him at least 10-15 songs of having a decent role. And of course the concepts/ideas, and maybe some arrangements as everyone else has been mentioning. As the band jammed on stuff instrumentally, MP would suggest trying a riff more often than this, or that. This is clear from watching studio stuff.
But progression wise MP had less input than JM or JR would have had is my guess, and of course nothing like JP. And that really is what makes songs appealing. I doubt MP was saying 'ok, during this section, lets do a B>G>D progression.' Sure, MP created probably the vast majority of the foundation of songs. But is a house a house with a foundation only?
Which is why MP comparing himself to Roger Waters as he frequently does is so far off. RW wrote basically all of The Wall, on top of many other songs... solely. RW probably did the lion's share of the progressions, and melodies. MP I'm guessing cannot claim the progressions part at all. So I find that comparison not fitting at all.
MP should aptly compare himself to Lars Ulrich, which is basically right on the money. Lars co-writes basically all the stuff, but I doubt Lars is suggesting chord changes; Hetfield does. Lars also was the main engine of that machine for a long time with legwork outside of the music, as was MP.
Until MP steps out like Phil Collins or Nick D'Virgilio, and actually writes a majority of a solo album, he will be Lars to me of the Prog world.