However, that did not mean that there were no issues with the technique. It was not perfect. Rui was already using the Hungry Pain technique on a permanently regular basis. After all, he had used the Hungry Pain technique to obtain a stronger Martial body that was grade two right off the bat.
Because he was measured to have extremely high stamina before he became a Martial Squire, his evolution process evolved his other physical parameters to a greater degree than at the cost of stamina.
Because he had chosen an all-rounder Martial body that was perfectly equally balanced between all parameters, the evolution process to reach that balance needed to evolve his stamina less than his other parameters because his stamina was already high, and disproportionately higher than the rest of his body. Thus by evolving his stamina, he could achieve the perfectly balanced Martial body that he had at the moment.