Note that the standard menu icon doesn’t scale either with the panel vertical size. Neither do indicators, running applications on the window list, or hide-to-taskbar icons. It seems to be an exclusive behaviour of only a small number of applets. I’d posit the idea that those are the ones having an inconsistent behaviour.
On the other hand, having icons scale automatically to the panel size is one of those undesirable features for many setups, particularly on vertical panels, because of the space lost on a direction that has a premium value on modern monitors.
I do prefer the idea of setting my own icon sizes manually, by replacing icon files, or through any preferences if there are any. Besides it better matches the desirable pattern of a configurable desktop. On that aspect, the advanced menu does it right.