The Camera|Camera's FOV must be updated to reflect ViewportSize|Camera/ViewportSize changes. The value of the FieldOfViewMode property determines which FOV value will be kept constant.

For example, when the value of FieldOfViewMode is set to FieldOfViewMode/Vertical, the horizontal FOV is updated when the viewport is resized, while the vertical FOV is kept constant.
On the other hand, if the value is set to FieldOfViewMode/Diagonal, both horizontal and vertical FOV might be changed to keep the diagonal FOV constant.