The G20 is a pretty long car. I'm not sure of the exact dimentions but it's significantly longer than a sentra. Plus the G20, well the P11 at least has extras like side impact beams, good sound deadning, side air bags, etc. those things add up.
The P11 is not that long , from memory I think it is 177in which is pretty short. That size is about the same as the new civic but the P11 is about 300-400lbs more. The reason I think is the side impact beam Jason said, and side airbag, sound deadning stuff, and sunroof, all those little things add up.
Well, if you had a P10.... naw... not gonna go there. A lot of the extra weight on G20's is the leather interior, sound deadening, nice features, etc... Also the auto tranny adds quite a bit of weight to the car. There is a suprising range between a base model 5-spd G20 and a fully loaded auto touring model. Go ride in a Sentra, then ride in a G20 and you'll hear and feel the difference right away, they are not the same car by any means.