Speed Twelve
It has to do with momentum I believe. If you are lighter, you won't have as much momentum when the rocket stops firing (and it doesn't last long, trust me!).
Not having done Physics since A-Levels many moons ago (when they still meant something, b'ah!) I can not remember the equation, but I am sure some kind soul will post it, thereby demonstrating my failing and often frail memory.
bootflap