The second wrong reason is the difficulty of manufacturing high-quality, optically-true lenses increases geometrically as the diameter of the lens increases. Additionally, the cost increases in proportion. With low-grade, large lenses, image distortion is very apparent around the outer portion of the lens. If a manufacturer stops-down the light transmission through the scope and reduces its effective aperture, the scope utilizes only the light that passes through the lens' center portion, where its optical distortions are least apparent. Manufacturers can then get away with a lower-quality, lower-price large lens. The user will notice no distortion (in store-counter light), because the outer portion of the lens is essentially doing nothing. Either way, the consumer gains nothing. If he paid a premium for the larger-objective scope expecting to benefit from increased brightness, he's being cheated.

Be An Informed Buyer
You can tell whether or not a scope is utilizing the full light-transmission properties of the objective lens by performing a simple test. This is the only test I know of allowing you to gain a sense of how a scope will perform in dim hunting light while examining it in store-bright light. You'll need a flashlight, plain piece of white paper, pencil and pocket ruler with millimeter scale.

Shine the flashlight through the scope from the eyepiece end while holding the notepaper over the objective lens. You will see a circle of light projected on the paper. This circle marks the effective aperture of the scope--the amount of light that can actually pass through it. Mark the edges of the circle and measure the diameter. If the scope is utilizing the full aperture of the objective lens, the circle of transmitted light should be nearly at its full diameter (don't be bothered by a one or two millimeter difference; a small amount of the edges of internal lenses and reticle rings must be used to hold them in place). If the diameter of the projected circle of light is noticeably smaller than the objective lens' diameter, the scope is being internally stopped, and the area of the objective lens outside the circle yields no benefit. If the scope is a variable, perform the test at the lowest and highest magnifications (the higher magnification is where cheating will be most evident). If the scope has any special adjustable light transmission features, test at the maximum brightness setting.

All other features being equal, a scope with a larger objective lens provides a brighter image--up to the point where the scope's exit pupil diameter gets bigger than the amount of light the eye can use. If you make an effective aperture test part of your purchase process, you'll ensure you're getting what you pay for.