With current high quality scopes, tube diameter has to do with adjustment ranges and nothing with lens quality or "light gathering". Yobuck is correct, divide objective diameter by magnification and you get exit pupil size. Average eye can dial ate to about 7mm, some people can get 9mm. The closer to 7mm the exit pupil the less strain on your eye. This is true when comparing apples to apples.
A few years back tube diameter came into play because only the top of the line scopes with the best lens and coatings had 30 mm tubes so naturally they seemed brighter and better. Now a days tube diameter is a marketing scheme and can be had with absolutely horrible glass.
To answer the original question, on low power you won't notice it and on high power I think lens/coating quality will limit you more than exit pupil.
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