if you only have 18 dreads....they are gonna be big fatties. The best way to judge mature thickness is on how big the sections are. The dread will mature to be aproximately the size of the section. Most ppl section off 1" or smaller. If your sections are bigger you might wanna think about redoing them.
It has a little to do with your hair thickness and texture .
My hair is really thin and really fine...so my dreads will always be a tad bit smaller than my sections. My hair was about the thickness of a dime when in a pony tail before i dreaded it. Now each of my dreads are as thick as a dime! I have about 30 dreads.
Most dreads will shrink up and fatten up a whole lot during the first year. So you can't judge the mature size by looking at a baby dread