Some of my saddles are 17.5s and some are 18s. It really just depends on the saddle and what feels right. I am very tall and average weight-wise, super long femur. I do not have a large derriere. Even with a long forward flap sometimes I just need the 18 to make the balance point work, but not always. Go by how the saddle rides and not the numbers.
Some saddles are just known to “ride small” too. My Tad Coffin, for example. Many ads for those will even say “18, rides like a 17.5…”
My Stubben dressage saddle is an 18 as well, I think dressage saddles in general ride small because they are more confining and if you have a long femur you’ve got to have some extra space. Even with the longer stirrups it’s an issue.
If you like the 18 best, get the 18 and don’t think twice about it. It doesn’t mean anything bad.