This ^^^. I'm just reiterating what others have already said, but it's something I think a lot of people have misconceptions about, so I'll emphasize a bit. The size of the objective lens, divided by the magnification, defines the exit pupil size (EP). So a 50mm objective set at 10x magnification produces a 5mm EP. This is the size of the beam of light coming through the scope and available to your eye. You'll notice 5mm is a lot smaller than the 34mm or 30mm or 1" tube size (and the size of the EP is unrelated to the tube size). Cranked down to 5x magnification, the EP is only 10mm.
So bigger tubes don't allow more light (*One caveat noted below). Manufacturers have three options when they make the tube bigger:
1) Increase the amount of space in the tube available for reticle movement,
2) Increase the thickness of the tube walls (toughness), or
3) Increase the lens diameter (*This is the caveat. Larger lenses won't increase the EP diameter, but they can increase resolution).
From what I see, most manufacturers seem to lean toward increased reticle movement and wall diameter (toughness).
The pupil of the human eye ranges from about 1.5-2.0mm in bright light up to about 7-8mm in near darkness. Compare this to the size of the EP, because even if your EP is 10mm, it's not doing your eye any good in terms of increased brightness (the eye can only accept what the pupil size allows).
Long-winded version of what others said succinctly: larger tube doesn't mean more light (but may improve other things).
