Created attachment 6357 screenshot Comparing to xfce4-mixer icon looks too big. Even other plugins' icons smaller (clipman, mail, dictionary, screenshot) moreover it doesn't use theme, it's black, but current icons (Mint-X) are gray.
If I understand correctly 355 line of file pulse-audio-button.c is guilty "else if (width >= 30 && width <= 40)"
This is likely caused by one of the following: - gtk2/gtk3 theming mismatch (you may try another theme), - symbolic (scalable) icons used in gtk3 (as a workaround you may try to reduce the panel size). The plugin uses the gtk3 library to save my effort when the panel gets ported to it. Hence there are some visual mismatches w.r.t. gtk2 plugins.
Strangely, but building this plugin with modified file pulseaudio-button.c Lines (from 351 to 358) if (width <= 21) button->icon_size = 16; else if (width >=22 && width <= 33) button->icon_size = 24; else if (width >= 34 && width <= 40) button->icon_size = 32; else button->icon_size = width; didn't make any effect, plugin icon still gets bigger when panel size becomes equal to 30. BTW I've created script for Mint-X users https://gist.github.com/7c664436de2e9db48acc.git
Not planning to fix this issue due to lack of time.