The bigger prop is used to take advantage of the electric motor's power band. They are good at making torque, and optimal efficiency is turning a large prop at a lower rate.

I converted a 1.20 Yak from glow to electric. It had a 1.20 2-stroke with 16x8, and for awhile a 1.10 YS 4-stroke with 16x6. Performance with electric is different, kind of a cross between the two. I am now running an 18x12. I think the extra diameter actually hurts performance of the plane, since the spiraling 'disc' of air flowing over the control surfaces is now larger and moving slower.

I think a 17" prop is probably the best compromise for the Venus - a 17x10 would be nice I think.