Pitch (i.e. the distance between two links/two cogs) is a major issue in drivetrain wear, which explains why waiting too long to replace a chain may lead to premature cassette replacement.
This being said, three questions:
1) What is the maximum chain elongation tolerated by a new cassette? 1.5%? 3%? Can you explain in some amount of detail? (intuitively I would think that the answer involves taking into account the difference in sprockets' radius at the bottom and near the top of a cog -- as chain pitch increases, it rides higher and higher on the cogs, up to the point where it is no longer gripped by the cog).
2) What is the maximum chain elongation at which a chain will skip on sprockets that have not been changed during the life of the chain? (intuitively I would think that the answer would be the same as above, or a tad lower - worn sprockets have deformed cogs, but the same principles apply).
3) Large sprockets last longer than smaller ones. Why? (intuitively I would think that wear is a function of the number of revolutions -- or distance/speed)