From what I can figure, a larger barrel id is more consistent because all balls are smaller than the id of the barrel. A small bore barrel will shoot a perfectly fitting or loose ball fine, but could have a velocity change if a tight ball is chambered.
From my expereience I have noticed that a barrel that is too tight, does not appear to have the accuracy grouping either. I don't have any data to support this though. The only thing a tighter barrel was used for years ago was to keep the balls from rolling out the end of the barrel.
A large ball may break when forced into a barrel that is too tight. This is quite often a combination of the bolt force and speed and the sudden slowing of the ball when it gets jammed into the barrel. Even the mighty level 10 would have trouble with this since the fat part of the ball doesn't enter the barrel until after the bolt is getting up to full speed and power.
Ideally we want a perfect match, but since paintballs are not perfect we accept the best compromise. Many players are willing to sacrifice a bit of "accuracy" to achieve consistency and minimize ball breaks. This is especially important in the tournament arena where a sudden hot shot can cost you a big penalty. So, a large bore barrel is used. The better, more consistent your paint is, the tighter you can get your barrel to the size of the ball without danger of getting the odd "tight" fit.
The ball sizers are designed to make a large bore barrel fit your paint so you have a snug fit to keep the paint from rolling forward while waiting to fire a shot. It is still advisable to not use a size that is too small for the paint or you will get barrel breaks.
Efficiency is affected by the size of the barrel. A barrel that is too tight increases the friction and decreases the efficiency. A barrel that is too loose lets too much air get by the ball while firing and is inefficient. So, again we try to get the best fit we can for best efficiency, compromising a bit for whatever perceived effect or problem we are trying to overcome or enhance.
Sorry for being long winded. Its hard to get all the stuff I want to say into a post and keep it short and readable.
Except for the Automag in front, its usually the man behind the equipment that counts.