It’s all about the jab. I have a longer reach than 80% of my opponents and long reach coupled with a strong jab gives me an advantage that allows me to stand up with fighters that are generally much more skilled.
The other thing to consider here is that reach and power are for the most part unrelated. A boxer’s punch gets its power primarily from the mechanics of the feet, legs, hips, and torso. This may not make sense, but the arms provide the least amount of power in a punch, if you ever watch a fight you will notice when a boxer throws a jab that his front foot pivots on the toes so that the heel points out, almost simultaneously his/her body turns slightly, and that is when the arm extends for the jab.
A skinny armed kid with good mechanics will always out punch the body builder who relies on only his arms for power.