As a point of fact, he was actually heavier post-cancer than pre-cancer. In form, Big Jan was of approximately the same weight. The actual figures for Armstrong's weight during his run was around 158-159. In form, Ullrich was right around that same number. Keep in mind that Ullrich is at least three inches taller.
This was trotted out, along with the higher cadence, as an explanation for the radical jump in performance post-cancer.
For what it's worth, Kevin Livingston, former lieutenant to Armstrong who defected to Ullrich's Telekom team in 2001, claimed Ullrich never rode with a haematocrit level above 42 during his time riding with Ullrich.