That is somewhat inaccurate.
The number of tracks and number of cars has nothing to do with performance. The only time these two factors would adversely affect performance is if more was being shown on screen. The "amount" of content a game has will have zero effect on performance if it isn't all being loaded on screen during gameplay. The only thing that would effect performace if both had the same content being shown visually, would be content that is not "visible", i.e., the cpu cycles needed to compute physics, etc..
Perhaps GT5 has better physics than GT5P and the "cost" to implement these physics meant making a sacrafice somewhere else.