The bookmarks bar content is centred when it doesn't fill the width of the window and I can't see a way of changing that.
I feel it would be nice to have an option to left/right align the content. For me, left alignment would not break my muscle memory when I add a new bookmark to it.
I acknowledge this problem will likely go away when I have added enough bookmarks to the bar, but I'm trying to keep it reasonably clean so it may take a while (exacerbating the problem as I have plenty of time to train the muscle memory between additions)