I don't really understand why anyone would wear 'normal' keeping pads under their trousers. This all started when a couple of county players were spied doing it in CB40 and 20/20 games......a few people then copied them because that's what people do...then they made up good reasons for doing it afterwards....of course, the real reason the county players did it was because they hadn't received their allocation of coloured pads from their sponsors and had no other option!
Fielding pads are, of course, designed to be worn under trousers and I imagine for most levels of cricket perfectly protective enough for a keeper to wear. If they're good enough to protect a silly mid on/off from a ball out of the middle of the bat then a keeper should have no problems. It's interesting that you don't see more moulded 'morrant' style keeping pads.....then again, I guess weight isn't such a big issue.