Well I cant relly find any sources about the exact weight of the Bulava. (Wikipedia states that it has a mass of 45 tones, i dont relly trust wikipedia facts) However the Bulava is supose to be longer then the Bark. Why this would meen that fewer missiles could be fitted on the YD I dont know.as I know Bulava is 37 tonns and Bark was something like 45-50 tonns. And constructors say that Bulava is little due to Bark
http://www.globalsecurity.org/wmd/world/russia/3m14.htm