Hello,
You can use custom CSS and make mega menu container larger
@media only screen and (min-width: 992px){
.item-design-mega-menu.columns-6 .nav-sublist-dropdown, .menu-element .item-design-mega-menu.columns-6 .nav-sublist-dropdown {
width: 1415px;
}}
But maybe it would be better to change column width for the item that is smaller and for the menu item that is larger http://prntscr.com/ikj8nw
Regards