Unless you are regularly trimming your hair, your braid will gradually become thinner.  The reason is, all of your hair is never the same length.  You will always have some hair that is still growing, then it falls out and is replaced with a new growing hair. 

People who have braids that appear to be the same thickness to the ends, typically have been trimming their hair to keep it at a specific length (and I would guess they have been doing this for years).  This allows a higher percentage of the hairs to get to the same length before falling out.

Glad to explain it to you.  Funny that you asked today.... I had braided my hair into 2 braids and wrapped them around my head this morning.  As I was braiding to the very ends, I was thinking, maybe it is time to start trimming to keep my hair at its current length and let it thicken up a bit.