Field of view is determined by the overall design of the individual scope, especially the design of the eyepiece.
Scopes designed for shorter eye relief often have a slightly wider fov.
2 different 3-9x40s often have different fovs.
A bigger objective lens does not mean a wider fov. 1-4x20s have wide fovs. A 3-9x50 does not usually have a wider fov than a 3-9x40.