I'm going to suggest that when you pressed and starched after piecing, the larger surface of the charms allowed for a little extra stretching. The narrow strips are bordered by seams, so there was not as much opportunity for the strips to stretch like the charms could.
If it were mine, I would lay the quilt top out on a flat surface, maybe even a clean floor if you don't have room on the counter or a table, and spray the whole thing down with a water mist. Let it dry naturally, and the charms should shrink back up to flat.
Then if you choose to press it again after it is dry, be very careful that you don't stretch the fabric while it is softened by steam or starch.