Hi!
After googling around for 2 hours I give up: How do I change the scale of the UI panels in 2.5? Somehow I managed to make all my buttons very big, which requires a lot of scrolling in the panels. 
How can I scale them smaller or alternatively reset the UI to defaults? Loading up my blender project seems to return the big scale every time.
Any help is welcome!
I think it’s a bug. Sometimes it just goes like that for no reason, but so far I’ve only experienced it with button panels when displayed horizontally.
I display all of my buttons (properties) panel vertically now, and I don’t think it’s happened since.
To autofit the panels to the correct size of your window, hover your mouse over the buttons window and press the ‘Home’ key on your keyboard.
You can also use ctr + middle mouse button to scale the panel to the size you want.
Thank you SO MUCH guys!