Some boxers grow into a bigger weight. Then they become a natural at a heavier weight than what they started at. At age 21 Mayweather was a world champion at 130 but he out grew that weight and 135 and 140 and finally matured into a full welterweight who could not longer go lower comfortably. Oscar eventually out grew even welterweight. It's unlikely Guerrero could comfortably fight at 126 when he fought Mayweather. Mayweather couldn't fight everybody that fans wanted him. He fought very strong opposition his whole career. No matter who he fought some fans would complain he should have fought someone else. He could never please everybody. Based on Mayweather's performances vs the performances of Williams and Margarito he would have been heavily favored to beat them.
Floyd Mayweather fought very few natural welterweights
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