Several valid questions you bring up. Bigger is typically better on a 50/50 scale. The point of a spotter is to see farther and save your legs from hiking 3 miles after what you thought was a monster bull/buck. In the same hand if your shooting for an ultra light pack setup you may want to slim it down for weight purposes. Even though I consider my gear on the ultralight side. I save weight everywhere possible in order to justify 10x42 binos, 15x56 binos and an 85mm spotter. (Yes typically all three on a 2-3 day hunt) I feel they all have their purpose but some do not agree with that which is ok. Another way to justify the sizes between a 50mm a 65mm or an 85mm is light gathering ability for dusk and dawn hunting and also typically the larger the scope the more zoom capacity it will offer. If you compare the zoom of a vortex razor 65 to the vortex razor 85 you will see what I mean.