Strange, I played with mine outside just before Christmas. It was cold (39 degrees F). All other balls (some regular, some pressureless from other companies) -- were all dead. I threw out a bagful that day. The tretorns were the only ones which bounced. They did not feel heavy. If anything, they were a little "too bouncy" -- that's the only complaint.
Anyway, I have not found anything better yet.
The other presureless balls just lose their bounce overtime. I prefer to deal with extra liveliness of Tretorns. It is almost like opening a new can in the middle of the day in the summer.