Change the content margins in Taskbar & System Tray > TaskBand > Basic > TaskBand (SmallIcons) > TaskItemButton
Those last two digits which add to 14 (7 top 7 bottom) + 16 (icon size) = 30 the size of the taskbar.
So you'd change the last 2 to 5. (4+4) = 8 + 16 = 24
This doesn't work with large icons and the smallest you can get with it is 32.
but only so I change the buttons size, the bar remains the same.