You are right there is something else affecting it, am trying to narrow it down, I will post again when I find what.
For me in my project then freshly dragged in Side bar with menu with no changes goes like this in smaller mode, basically the top div in it there goes to almost nothing
It is likely that this is some kind of conflict with other CSS in your project. You can try disabling your stylesheets one after the other to narrow it down.