sew through a loop, not the actual fiber itself, which will weaken the strand and possibly cause a tear.
Pick up one loop and run the needle through, then go over to the other side for the corresponding stitch and grab one loop, working your way down (I like to seam from top to bottom on a flat surface) so that after a few inches you can see the sewing yarn making a diagonal pattern between the pieces.
Here are a couple of examples...this is a free-form shawl/scarf I made earlier this year. Basically you just crochet some motif patterns and piece them together on top of a paper template you've made. Showing the right side where I've left some edges free to add texture...
This is the reverse side where I seamed a couple of pieces together. If you look closely at the gray piece on the left side, you can see the diagonal lines joining it to the white area to its right...through one loop, then back to the gray side through one loop, then back to the white side...one loop, etc.
The sides abut perfectly, and there's no bulky seam.
For this I didn't use seaming pins, but little pieces of yarn to loosely tie one edge to the other. I did this for as large an area as I could, matching and tying, then sewed and removed the little ties as I went.
My next free form project will probably be done in one color only, to highlight texture more.
Anyway, the problem with bulky seams goes away when you match the seams side by side instead of right sides together...