Change icon sizes of vertical menu?

[linuxy]

Hi friends.

I've setup my main toolbar and it looks great, with large icons (2K monitor).
But I've noticed that any submenu/vertical bar I create also uses that large icon size, and this looks quite bad and wastes so much whitespace.

Can I please change the icon size of the subbar / vertical menu without changing the main default toolbar icon size?

Thank you kindly.

[linuxy]

Here is a screenshot. Can you tell me how I can change the sub-bar vertical drop-down icon sizes, make them smaller and make the menu more compact?

https://i.ibb.co/52zcB1Q/example-of-large-buttons-more-compact-please.jpg

Thank you.