Thanks for all comments and hints.
The built-in icons are all too large for my purpose. I use a custom icon with plainer style instead:
bb = Button Box ("");
bb << Set Icon( "plus.png" );
with this small plus.png icon
results in quite exactly the small "add" buttons I need in my list:
![JWi_0-1585812519652.png JWi_0-1585812519652.png](https://community.jmp.com/t5/image/serverpage/image-id/22731i3D68EF7324D73CD5/image-size/small?v=v2&px=200)