Bigcountry, Sorry, but I will rely on the optical experts from Leupold. This is directly from their website:
The Leupold Answer Guide
Advantages of a 30mm Maintube
Does a 30mm main tube give you more light? The principal advantages of the 30mm tube are added strength and increased adjustment range for windage and elevation. For example, the M8-12X (1" main tube) has a total elevation adjustment of 51 minutes. The Mark-4 M1-10X (30mm main tube) has 90 minutes. The percentage of light passing through a scope is a function of lens coating and optical design, and has nothing to do with tube diameter.
I assume that Leupold does have some valid expertise in scope design.