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They also said in the Chunin exams arc that, in order to compensate for a reduced headcount, the Sand had to increase the quality of each individual shinobi.
The Sand aren't stronger than the leaf. It's actually quite the opposite. If you pay attention to the Chunnin exams arc it becomes very clear. Gaara and his siblings are the children of the 3rd Kazekage. Naturally they will be stronger.
Remember, the Sand shinobi we're seeing: Temari, Kankuro and Gaara are offspring of the most powerful clan in the hidden sand village. They're the late Kazekage's kids.