Yep, they'll definitely thicken up. See, all those hairs that swirl around in gross clumps in the tub after your shower will still be falling out of your head every day, BUT they're just staying in the dreads. Therefore, after a few months or so, you'll notice that they are starting to get thicker, and this is the reason why. If you prefer thin dreads, start thin and they won't get huge or anything, just a bit fatter. You can always try cutting them down the middle if they get too fat. But don't start too thin! Hair is always breaking and retangling and regrowing, and you don't want your dreads to be so thin that they start breaking off, eek! Just use your judgment; one finger wide is usually a good guide.