Here are the two RTP's that I have. The first is an older HO RTP (I think). It has some padding under the foot and it appears to be holding my foot pretty well from a side view. However, looking from the back, there are large gaps that just don't conform to my foot very well. Even after 4 or so years of skiing with RTP's, this is not confidence inspiring and it's not easy to get my foot into.
The second is my Radar ARTP. It looks like it has a lot of padding but really it's pretty thin padding under the forefoot and heel and actually a lot thinner than the HO RTP above. The rest is really just there as support to keep the foot from sliding around. Looking from the back, this RTP fits more like a shoe and completely conforms to my foot. So not only is it comfortable like a full boot, it's confidence inspiring as well.