There are three different toolbar sizes so that you can display different buttons at different viewports.
As you’ve indicated you’re using XF 2.1 you can control the toolbars for narrow displays under Admin CP > Content > BB Code button manager.
Note it’s only the “Extra small” toolbar that has any buttons removed. It’s mostly so that it doesn’t break to three lines of buttons on some pages. That can reduce the amount of vertical space on a small screen by quite a bit.