Thank you Derek! I found this thread because suddenly my taskbar buttons In noticed were larger. I must have changed something inadvertantly. But I didn't know how to fix it. Your suggestion of reducing the caption buttons fixed the problem and returned them to how they used to be. Many thanks!
Assuming your taskbar is already set for small icons you could try this:
Right click desktop > Properties > Appearance > Advanced button.
In the Item box select Caption Buttons. If you reduce the size figure it will reduce the size of both the Taskbar and the Active Title Bar (they are linked). As a by-product the size of the icons in the taskbar will reduce too.