Going back and watching Wetton vs someone like Chris Squire on bass with a pick, is like watching a ham-fisted player and a very graceful, gentle pick player.
Wetton just beat the living crap out of the basses he used, fingerstyle or pick, while Squire "coaxed" the tones out of his - even his more aggressive sounds.
Squire uses a pick which is considered "thin" by Carol Kaye, and he still plays pickstyle, and seemingly effortlessly and painlessly.
My point is, I think there's more to Wetton's story than thin picks...his "style" of picking was more to blame(as also has been mentioned here). His pinky and ring fingers anchoring on the bass(in the UK vids I have seen) always looked awkward to me.
I'm not saying Jimmy M isn't right about how he feels, b/c I can't go much lower than 1.00mm and feel like I'm losing economy of motion, but I also feel the really thick picks lose some attack from all the material that has to pass the string.