Created attachment 7146 Vertical The icon of the Whisker menu has been resized to too small. In other words, the left and right padding (empty space) of the icon is too much (and is not proportional to the icon width). Vertical mode, row size is 22 pixels, appearance style is Numix, icon set is Elementary Xfce Darker. I already posted this issue to the whisker plugin ("not the right place").
Sorry I only read the last line of your comment after moving it to whiskermenu. My guess is that Numix sets some padding on buttons by default and that causes the icon to be scaled/so small. So either teach Numix to add less padding (theming issue) or let whiskermenu override all theme settings with some padding settings of its own (which it may currently be doing already, not sure). Anyway, if Graeme doesn't consider it a whisker issue it's most likely a Numix (theming) issue.
This doesn't happen for me. I just tested with Whisker Menu 2.1.7 and Numix, and the icon does not shrink to nothing on a 22-pixel vertical panel. I also fired up a Xubuntu 18.04 virtual machine, and it doesn't happen there, either. I don't think this bug happens anymore. I am going to mark it as done.