<< But they are using the same platter isn't it ? so evern smaller size the data will store closer to the outside edge of the disk too right ? cause same platter. >>
same type of platter, yes, but with more sides/platters in use. say you have a 40gb hard drive with 1 platter and 2 heads (one on each side). if you were to put 40 gigs of data on it, you would fill up the whole drive all the way to the inside diameter. if you had a 120gb drive with 3 platters, that same 40gb of data would be distributed across the outside diameters of the 3 platters meaning that that data would spin by the heads faster than they would if the data was on the inside.
dunno if that makes sense....
basically, it comes down to where the data is being stored and that outside locations give you better transfer rates than if the data were to be stored on the slower interior.
not to mention there are seek times to be factored in on the larger drives. now i dont know how much the extra heads on the arm affect the times (not enough to show in the specs), but theoretically on a larger drive, if you have the same amount of data on that and a smaller one, the larger drive will give you better seek times. the arm will not have to move as far on the larger drive, as the data is in a smaller area on the axis on which the arm moves on. if your arm was on the outside of the platter/drive, and it was asked to acess the 40gb mark on a 40 gb drive, it would have to move all the way to the inside, whereas on a 120gb drive, it might only have to move 1/4th the way.