According to the individual at Leupold who responded to my email, the 30mm tube allows a greater range of adjustments. The light transmission is determined by the objective lens.
Actually the light gathering
is determined by the objective lens, but at the point where the internal adjustments kick in the "pencil of light" from that lens has been tightened down to about 1/8" (3mm or so) so adding diameter allows more movment of the internal tube which is how you adjust your crosshairs.
Think of it this way. Draw a 1/8" circle on paper then draw a bigger 30mm (1 3/16") circle round it. Now draw a 1" (25.4mm) circle inside of the 30mm one, but outside the 1/8" one & you'll see how much more movment is possible for a scope with a 30mm tube as opposed to a 1" tube.***
is governed by design of the glass, optical coating efficiency & so on.
*** yes I know, these are approximate, but it does give a visual clue to how optical performance works. the 1/8" beam of light thrown back onto the internal tube is way smaller than either of the other 2 measurements, but it can "wander about more" in a bigger circle..