It does actually vary....on amazon now they have started pointing out that some of their sizes are "asian" and therefore much smaller than the same UK size would be.....also I wouldn't call a size 18 very big!
Many years ago my late mother worked in Ladies Apparel inside a Department Store. When measuring customers who had a 36" chest she would often remark to the Junior that "Madam has one yard of bust."
There doesn't seem to be any standard conversion. I am size 22 and this is often XL. When I shop on the internet, I always look at the size charts for that particular site. Incidentally, I have NEVER had to buy 3XL!!! However, I have sometimes seen 18 as XL, but it is unusual.
Depends where you shop 5xl on some Chinese sites is a 20
In Germany and the U.S. about a size30. Uk it's 26 in most places. I bought a 5xl jacket from a site based in the Philippines and it measured up a 14, best way to think about it is look at the size of the women in the country of origin places where they are usually tiny dainty folk generally have very small sizes so a 12 would be a very large size to them.
it seems to me that different companies size clothes differently even though they have a size number. I have several items from one company that are labelled with different sizes but they all fit the same. go figure!