I see where you're coming from but there are a lot of factors involved. For example, in a 4l60 (which really means anything based on a 700r4 i.e. 4l60, 60e, 65e, 70e) the only difference between 3rd and 4th is that a band (band brake) applies. The band STOPS the drum it interacts with. So in that case there is actually less weight spinning in 4th then there is in 3rd. I doubt the weight of those spinning parts is a major factor, just illustrative of unintuitive it is trying to picture how a planetary-gear-based auto trans actually works.
It's undeniable physics that a 1:1 ratio is inherently more efficient than an overdrive ratio, but how those ratios are achieved can easily outweigh that. One problem with making your top ratio 1:1 with planetary gearsets is that you can make 3 ratios with a planetary geaset. An underdrive, a 1:1, and an overdrive (and a reverse but that's irrelevant unless you are applying one reverse to another reverse to get back to a forward gear). So already you realize that in order to get 4 forward speeds like a 4l60 you have to have more than one planetary gearset (a 4l60 has two sets). Then, you realize that if you wanted to have 4 gear ratios BUT with the top ratio being exactly 1:1, you actually need THREE gearsets with all their associated spinny bits. Now you can feed one gearsets underdrive ratio to another gearsets overdrive ratio or vice versa and end up with 1:1, but that's not giving you the benefit you were referring to anyway. So with planetary based automatics you can always just bind 2 of the 3 elements of all the planetary sets and end up 1:1, but to have sufficient gears under 1:1 (called the trans's 'ratio spread') would in some cases require more parts and weight and friction and loss then having the same number of ratios, but with the top ratio being overdriven.
