First, Tommy Hearns had a longer reach than Ray Leonard, but Sugar beat him. Why? Because Leonard's skill level was better than Hearns'.
Boxers and MMAs face guys with longer arm length all the time. And beat them. Why? Because their skill level and ability to adjust.
Second, yes, there is a reason why even wrestling has weight classes. Same as MMA. It's because WEIGHT MAKES A DIFFERENCE!
Demetrius "Mighty Mouse" Johnson was one of the best pound for pound fighters in the UFC. He's not beating Connor McGregor, who isn't even a ground fighter.
Third, if the big guy is just as skilled as you are, you're not beating him. Sorry.