Snyper,
Ocular lens diameter has a LOT to do with FOV, because it's essentially the screen where we view the world. Magnification and eye relief are also involved, but aren't the entire equation.
A quick (and very inconclusive) look on the interweb looks like it has more to do with eye relief than ocular diameter.
-SMALL ocular
Leupold 6x42 FX-3 Riflescope Specifications
Field of View @ 100 yards (ft): 17.3
Eye Relief (in): 4.43
Schmidt & Bender 6x42 Klassik
eye relief (in): 3.1
Field of View @ 100 yds (ft) 22
-LARGE ocular
SWFA SS 6x42
Field of View @ 100 yds (ft) 20.1
eye relief 3.5
Meopta 6x42 MeoPro
Field of View @ 100 yds (ft) 20
eye relief 3.5.
