How Gallery takes on the Width!
We are not limiting width of the Gallery, hence, Cozy Gallery takes the 100% width of its container. However, most themes have limited width page variant by default this makes Gallery looks narrower.
Solution
If your theme supports the wide – page variant, then simply switch to the variant and your Gallery will take full width.
What if you don’t have such template?
However, in case your theme doesn’t support wide-width variant, you can try contacting theme developer. Since all themes are different, some just can’t support the full-width page because of the way they are designed. In that case only theme developer is able to help you best.