David, there are multiple answers to your question so I'll give it my best.....for one it depends on which handles you are talking about as to the thickness of the handle. With the stabilized wooden I handles it's true I did make them thicker but there were multiple reasons for that...number one is yes you are correct that I myself like a handle that's not too thin. I have large hands and for that reason I like a thicker handle; especially on a fixed blade knife that I really want to have a good hold of. Second, it was very hard to make the handles thin on the stabilized wood with the corby bolts I used because the finger grips that I usually would have contoured in were right where the corby bolts were, mainly because of the configuration of the holes that spyderco decided to use. Third, on the stabilized wooden handles my opinion is that although they are very, very durable the thinner you make them the weaker they become. For everyday use that's no big deal, but if you were to drop the knife out of a tree onto ,say, a hard surface like concrete, the thicker they are the less likely they are too crack. As far as the interchangeable handles I have done (mainly from G-10 and micarta) they are actually quite a bit thinner because the way that the screws fit into the hole configuration works better with the finger contours. That being said I can't make them too thin because they have to be thick enough to be held on with the torx screws and pivot pin; the more threads in the pivot pin (or the longer the pin), the better the handles are held on. So to answer your question I guess personally I do like somewhat of a thicker handle but to be honest none of the people that I made handles for really had much of a request as far as handle thickness so I just made them about what I would call an average size handle. And of course I had to work around the hardware with the holes spyderco chose to place. I could definitely have made them thinner to some degree as long as they still worked with the hardware, but I was getting nothing but good reviews from the owners so I decided not to change anything. Hope that helps!
PS. The pic of my personally knife with it on the spine does overaccentuate the thickness of the bottom part of the handle because of the way that I took the picture, but you are correct that it is definitely a good amount wider at the bottom. Fits my hand like a glove though!