Field of view (FOV) is measured in feet at 100 yards. This is the amount of view you see through your scope from right to left at that distance. As magnification is increased, FOV goes down. As magnification is decreased, FOV goes up. For instance, a typical 3x variable scope might have a FOV at 100 yards of a bit over 30 feet, and at 9x, the FOV would be around 14 feet. A larger objective lens diameter will not change these figures.
Field of view is directly related to the construction of the eyepiece.
Scopes with a wide field of view allow you to see more of your surroundings and help you take more accurate and efficient shots while hunting. When you have a smaller FOV, you may be too focused on a single target to notice there's a better opportunity nearby. If you want a wider field of view from your scope, it's best to buy a variable scope with a lower magnification range.