I had problems with width, it was related with some extra spacing divs in wordpress theme. I had 100% width, but some spacing left anyway on the left and right side of the gallery images.
Try to check browser console. Maybe you can see there elements which block width of the gallery, in your “box”.
yes, the main trick it’s make sure that you have full width defined in gallery settings. In the case if blog theme will not have any extra spacing then your gallery will be properly aligned with 100% width
One more tip: You can use browser console to check some spacing and general gallery page structure. If you wish you can post here link to the gallery page and we’ll help you to find solution.
