Khabib was a 23-year-old welterweight when the UFC signed him. His father pushed him to try to fight at lightweight in the UFC to protect the undefeated record that he had carefully engineered for him. One of the tenets of Abdulmanap's teachings was to select opponents over whom you have an advantage in weight or wrestling skills.
When Khabib was 11-0, Abdulmanap matched him up against a fighter with an 0-2 record. When he was 14-0, Khabib's opponent was 0-0. After signing with the UFC, this strategy would have been near impossible to pursue, so Abdulmanap insisted on Khabib doing a death cut to lightweight and shying away from MMA fighters that used wrestling extensively.
Khabib spent his entire tenure in the UFC struggling to make weight. He missed weight several times, repeatedly pulled out of fights, was a no-show at weigh-ins, was often injured, averaged barely 1 fight per year over the last 7 years of his career, and got to a point where he was only able to fight in Abu Dhabi thanks to their sham weigh-ins.
Khabib has continued to gain weight in retirement. He will almost certainly never fight in the UFC again because he will never make lightweight again. His father stressed to him that having an advantage in weight and wrestling skills over his opponent was critical to creating an uneven playing field to protect the undefeated record.