It's a knitting metaphor that had me laughing—because it's also involves a clever pun.
When I start telling things, they’re not so huge, but the more I tell, the bigger they become.
So, "I'll promise to cut down the size of my yarns a bit."
2: a narrative of adventures; especially : a tall tale · a roaring good yarn
1 a: a continuous often plied strand composed of either natural or man-made fibers or filaments and used in weaving and knitting to form cloth
But with knitting, there are also seams:
1 a : the joining of two pieces (as of cloth or leather) by sewing usually near the edge
b : the stitching used in such a joining
It's the word yarn that's the pun—because it's being used both to mean
her stories and to act as a knitting metaphor (which also involves seams).
Taking all of that together, what she's actually communicating is that she's known to tell tall tales, but she'll do her best to join all of the pieces of the next story together for Tess—making it more concise and understandable. (And perhaps more honest or believable.)