It's pretty simple actually:
When you are the bigger fighter, you are supposed to win.
A smaller fighter eating more and cutting less weight doesn't make him equal size to the fighter a weight class above.
When you are supposed to win easily and the smaller fighter "almost" (if not actually) beats you, it is nothing to bragg about.
In short: The LW champ who is #2 p4p is supposed to beat a FW coming up to his weight class fucking easily. If he struggles to the point where people can debate who won the fight, it is embarrassing and definitely doesn't make him p4p #1.
If you disagree with the above it should be no problem for Islam to move up to WW and have a very, very close fight with the WW champ. He is in his estimation #1 p4p after all. I am excited to see him challenging the winner of Leon vs Usman next - hold your breath!