Presumably it's SFP so the distance between dots changes as the power is changed thus better accommodating various velocities. The size of the objective bell only increases the field of view. It's the size of the OCULAR objective that affects light transmission. Nice scope but parallax set at 75 yrds seems more suited for rifles ... or target shooting way out there.
I just happen to think that crossbows are for up close and personal. If I want to stretch my game there are half a dozen scoped 'hancannons' as well as deer rifles at my disposal. But hey, whatever turns ones crank, eh?