well the 12" powerbook g4 is 2.1kg, while the 15.4" one is 2.54kg.
The difference only 440g.

If more than 700g difference like IBM Sony or Fujitsu, then only worth to get the 12", otherwise for another 440g, I will choose the bigger one.

I think that's the reason Apple cancelled the 12" model for core / core2 duo.