While I agree that the Castelli Kiss is thinner than the X2, IMHO it's nowhere near as good.

Another pad that it a bit thinner is the Endura Equipe Crit Pad.

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Snacking on carrot sticks - Where did it all go so wrong?
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Finsbury Park CC

I like the DeSoto Riviera chamois best - their 400Mile is too bulky for my taste. In fairness though, I don't put in nearly the mileage of most here.

Well, "good" is rather subjective for something like this. I bought the Castelli Body Paint Bib shorts and while the bibs (fabric, fit) themselves are fantastic, the seatpad was too bulky and wide for me and caused chafing.

I recently bought Castelli Body Paint Tri shorts and those have a "Tri Kiss" seatpad that is really thin. I asked Castelli whethe they could make cycling bibs with that seatpad, it seems some people would like that (maybe those people should also contact Castelli).