I use a 50mm scope on most guns but do have a couple of 44mm and 40mm scopes as well. My older 30-30 has a 44mm which works great for me. I also prefer a 3x9 or a 4x10 scope for deer and usually keep it set on around 5x in the stand. During morning hunts the objective is really not a big deal but the evenings can be legally extended by 5-10 minutes with the 50mm scope. I always sit on stand until it is too dark to make out leaves on the ground. I also prefer to be in the stand before I can make out leaves in the morning.
I can see how it might
seem to work the way you describe, but there's a crucial catch - there's a limit to how much light your eye can let in. Because of this, increasing the exit pupil of the scope to more than 7mm
makes no difference. At 5x you get a 7mm exit pupil with a 35 mm objective lens, so a 40mm would be more than adequate. Using a 50 mm lens only adds weight, cost and raises the scope higher, which is already a sore point on Marlins because of their low stocks designed for iron sights.
From:
http://www.shootingtimes.com/2011/01/03/optics_opticpupil_061907/#ixzz20oaTffoV
'You can calculate the size of a scope’s exit pupil by dividing the effective objective diameter in millimeters by the magnification. For a 4X 32mm hunting scope, divide the 32mm objective size by 4 and you find that the exit pupil is a generous 8mm in diameter. With a 6.5-20X 50mm target/varmint variable scope, the exit pupil ranges from a large 7.7mm at 6.5X to a smallish 2.5mm at 20X. In a low-light situation, all other factors being equal, a lower magnification setting will provide seemingly brighter viewing than a higher one.
It is tempting to conclude that the largest obtainable exit pupil is the most desirable. But that’s not always the case.
The catch is that the pupil of a normal human eye opens to a maximum diameter of 5mm to 7mm, depending on the individual, even in extremely dark surroundings. Exit-pupil diameters that exceed about 7mm deliver more light than your eye can accept."
Personally I find 4x plenty of magnification for deer as far as you have any business shooting at them with a 30-30, so I'd go for a 4x with a 28 or 30mm objective, mounted as low as I could get it.