The only time a fat Bono surprised me with his voice was Union Chapel...he was large and in charge at that time, and he sounded great....but that wasn't on tour...on tour, fat Bono gets out of breath faster and that effects basically every other aspect of the show from Bono's perspective...makes it more difficult to simply hit certain notes.
This is definitely dependent on the singer. Some er, "larger people", such as the late Nell Carter, have stated that their voices are far more powerful with extra weight than without. It's with that in mind, that I feel Bono's voice would probably be the same and any extra weight would NOT make it more difficult to hit certain notes.
That said, having seen U2 four times recently, Bono did not look heavy at all. Keep in mind, save maybe the "Boy", JT and "Pop" eras, Bono has always had a little extra pudge (see shirtless photos from the TUF era and Bono self-admitted to being heavy later in the ZOO TV era). Given the shirtless photos we've seen of Bono recently (swimming in France), I would say he has definitely lost weight and is looking rather good.
Furthermore, while his voice can sound tired or strained on any given night (also true during the JT era - notice on the Paris DVD that came with the 20th anniversary of JT, Bono was unable to hit notes in some songs), he has been sounding really strong. I left the Boston 2 (or Foxboro 2) show a bit early (in a vain attempt to beat traffic - phailed!
). One advantage though is that as I walked along the GA floor then outside the stadium, I could hear U2 - especially Bono - better and better. And without the reverberations of the music, I clearly heard his voice soaring for songs like WOWY. It was arguably the best WOWY I've heard since the JT/Lovetown era!
Therefore, while he may still have a few extra pounds, this is probably his "true" form. He's always been a bit pudgy with a few exceptions (and he probably really worked out to get to those levels in the past). Given his age (and I know I can relate), I contend he's doing well.