Try the 'Tween' Behavior. It lets you trigger smooth or exponential (or like 50 other) adjustments to things like size, opacity, color etc.
ex:
Condition: Mouse Cursor is over Object (Button)
Action: Tween (Two Properties) (Size) Over X seconds
- X (width)
- Y (height)
Else: Tween (Two Properties) (Size) to Default size Over X seconds