What is the meaning of "if the uncovered part of the head is excessive" when wiping over a khimar, which leads to the invalidation of purification?

If a small portion of the head is uncovered during the wiping (masah), it does not cause harm. However, if a large portion is uncovered, considered excessive by custom ('urf), then it does cause harm and affects the validity of the wiping. "Excessive" here means an increase and a large quantity.