How Thread Sizes are calculated:
No.1 yarn is 1 pound of flax spun to a length of 300 yards.
No.2 yarn is 1 pound of flax spun to a length of 600 yards.
No.18 yarn is 1 pound of flax spun to a length of 5,400 yards.
Taking for example a 18/3 thread:
The "3" refers to the number of yarns that have been plied together.
The "18" indicates that it takes 5,400 yards of such yarn to weigh a pound.
It can be seen why bigger numbers mean smaller thread -- the thinner the yarns, the more of them it takes to make a pound.
A 25/3 thread is made of three yarns, each made from 1 pound of flax spun out to 7,500 yards and then twisted together.