80mm or larger lens allows a slightly higher magnification power to start with. For example, you will see 15-45x60mm and 20-60x80mm scopes. When designing a scope, we try to make the exit pupil above 4mm (exit pupil= objective lens size/magnification power) so it can gather enough light. That's why we can start with 20x for 80mm lens, while keeping it at 15x for a 60mm lens. You just have to balance the weight, power, light gathering capability when you make a decision on which one to go with.