Why does the sashiko stitching cause the fabric to tighten, pucker or bunch up?
As your stitching progresses you are adding a lot of fat thread in between the threads of the fabric. This distorts the fabric surface forcing it to bunch up. Leaving some loose or slack thread on the back of your stitching that can be pulled into the fabric when it tighten up will counteract this.
Most the of the slack or loose thread you leave on the back will be pulled into the fabric as your stitching progresses, and you may not see it anymore by the time your project is completed.
Study these photographs for a bit. (Ignore the fabric in the photo - your fabric will likely look different, it doesn't matter for this discussion).
You can see I have left the thread quite loose where I have carried the thread from one line of stitching to another.
But it is also important to leave little loops of thread in the stitching lines themselves. I suggest about a quarter inch every six inches or so as a rule of thumb.
Another good rule of thumb is to leave about a quarter inch loop whenever you turn a corner.
The more dense the sashiko pattern you are stitching, the more important this is.
You already know to smooth your fabric out after every needle full of stitches to flatten the fabric.
It is also a good idea to stop stitching every so often and tug the fabric firmly both side to side and diagonally. This has the effect of pulling the little loops of thread you left on the back of your stitching into the fabric so can stay flat.
This dense Sashiko stitching style is called Hitomezashi or single stitch Sashiko. This style of sashiko really needs to be able to pull in extra thread as the stitching fills more of the fabric. You want to be extra sure to leave slack threads on the back that can be pulled in to the fabric later.
This large pattern (moyosashi) sashiko won't need as much looseness in the threads, but will still need some.
Can you leave too much slack thread on the back? Not Really. Sashiko thread tends to stay where it's put :-) It isn't likely to loosen up on the front after it has been washed once (or even before that). And if that did happen you could simply pull it back onto the back of the fabric. I have only ever had it happen if I snagged the stitching on something - and it was easy enough to pull back to the back.
What to do if you didn't remember to leave enough slack and your fabric is puckered? Cut a thread or two on the back and ease the stitching /fabric flat, then restitch the area where the thread pulled out.
Enjoy your Stitching,
Susan