The following is a paraphrased quote from Adam Del Monte.
You can hear exactly what he said at 9:20.Adam Del Monte wrote: The other thing you want to work on is crossing strings (by crossing Del Monte means any movement to another string). That's where the real trouble begins because at high speeds when you are changing height of strings you are going to be making a very drastic distance. On a single string you are dancing on a pin, and in moving to the next string this is a very big distance in the right hand guitar world. Believe it or not you are traveling twenty times further or something like that. OK you do the math.
Also IMO if you have small hands and keep your thumb anchored on the low E-string when playing scales (as Del Monte and many world class players do) then playing on the high B-E strings can be traumatic unless the string spacing is reduced to something reasonable. At the very least it's obvious that in any passage, the hand shape and angles of attack are more consistent with reduced spacing.
Edit: By consistent hand shape it was simply meant that the ideal hand shape that you find in many method books is destroyed when the space between the thumb and fingers is too great. A small handed person knows quite well that with thumb on low E and fingers on B or high E that the hand flattens out considerably, i.e., the distance between the big knuckle and finger tips becomes much greater. You don't even have to have small hands for this to be of note. Segovia kept his thumb just 2 or 3 strings behind the fingertips to keep a consistent hand shape. There's a whole school of thought that says Segovia's technique is the way to go. I believe Richard Provost is one of its promoters. Reduced string spacing automatically makes hand shape more consistent as a simple geometric fact, and this obviously is of note more to the small handed than the large.