Icons in toolbars such as QuickLaunch usually remain at a constant size of 16x16 pixels, however some skins may have adjusted the margins between icons which may give the illusion of smaller icons.

If you want to modify the margins (in SKS6) you need to go into Edit Start Menu & Taskbar then click on QuickLaunch Icon Button from the left-hand-side list, then click on the Text tab and modify the content margins.
It's best to work on a copy of the original skin in case something goes wrong but once you're happy with the result you can remove the original.