I am so relieved to see that others are having this problem too. It is so irritating to put time into creating a great description, carefully photographing items, etc, etc....only to see that the final product is TOO NARROW. Irritating!
I followed another poster's advice to remedy the situation. Here is how:
1) go to your Store
2) find the link "Manage My Store"
3) In the left column, find "Store Marketing", click on "Listing Frame".
4) Under "Store Header Features", select the button that says "Store Header Only", NOT "Store Header and left-nav. category...".
I believe this will work. Good luck,
Lori