I was having this same issue on a 7.1 site with a centered nav. I wrote this CSS and dropped it into the Custom CSS section which seems to solve the issue.
// Header Nav Width Adjustment
.header-layout-branding-center-nav-center .header-title-nav-wrapper {
flex: 2 0 100%;
}
.header-layout-branding-center-nav-center .header-actions--right {
position: relative;
right: 4vw;
}
The first value in the flex property basically tells the div holding the nav to grow to fill the space. However, this seemed to push the right side action buttons (cart icon, in my case) off the screen a bit, so I added the next line of code to move it back into place.
A little bit of a hack job, but gets the job done.
