When the UFC first began fighters were not as skilled as they are today. Many of the fighters had little to no grappling skills and many of the traditional arts had little full contact stand up skill. During these first events, the fighter with the most full contact experience, either stand up or grappling, generally won. Now that MMA has evolved the weight classes are necessary because if you have fighter A at 155 lbs and fighter B at 205 lbs and both are of about the same skill fighter B will win. It's a matter of physics. If you have an unskilled 205 lber against the skilled 155 lber the smaller guy has a much better chance but size is always a factor when it comes to fighting.
"The hero and the coward both feel the same thing, but the hero projects his fear onto his opponent while the coward runs. 'Fear'. It's the same thing, but it's what you do with it that matters". -Cus D'Amato