That is set inside the program itself - the hotBar elements are drawn as graphic objects and the text is a "helvetica" set to size 8pt. The issue is that GTK thinks the screen is 96dpi, but it is actually about twice that.
You may notice that the hover text (pop-up) is at the same font size as the menus which you fixed. So my simplistic approach is to set the labels off (Options->Display->HotBarLabels) which often gets more objects shown on one screen in the hotbar anyway and hover over them when I want to know what they are...
This is not perfect, admittedly.
When we have got to GTK3 in V6 there will be more system options for controlling scale and dpi (and the GTK code is better about handling Hidpi anyway).