It's also to protect the fighters from sustaining a few more injuries than what you are already opening yourself up for, I know when I spar someone who is 20kg heavier than me, you can feel a massive difference in power even if you are both going at 40% of your output. Even Checking kicks can end in unnecessary injury.
Although I don't mind clinching with inexperienced heavier people, its fun to throw and knee those who don't know how to use their weight
Agree with ninearms. Nothing harder than fighting someone who has the same range and weight as you! Makes for a more interesting match.