Other factors determine field of view... THe size of the erector cell, the focal length of the prime optic, the physical size of the reticle frame, and mostly the eyepiece, and eye relief... the more eyerelief, the less field of view.
Ok.
Vari-X III 4.5-14x40mm Adj. Obj. Tactical
F.O.V. FOV @ 100 yds (ft): 20.8(4.5x) 7.4(14x)
Vari-X III 4.5-14x50mm Long Range Tactical
F.O.V. FOV @ 100 yds (ft): 19.0(4.5x) 6.0(14x)
According to Leupold site, the smaller Obj has a bigger FOV than the larger Obj, however the 40mm has a tube dia of 1 in and the 50mm has a 30mm tube. So unless you can find a 40 mm with a 30 mm tube and compare it to a 50 mm with a 30 mm tube what I said may or may not be correct.