Hey Sebastian,
Some behaviors stop affecting their target parameters after the behavior ends…and some don’t. Throw, for example, affects the Position parameter of an object, but the object doesn’t jump back to its original location after the Throw ends. Grow/Shrink, as you’ve noticed, does stop affecting the Scale parameter when the behavior ends.
There are a couple of ways to get around this. As Noah suggested, simply extending the lifetime of the behavior will keep its affect in place. In the case of Grow/Shrink, though, the behavior is constantly driving Scale, so extending the behavior keeps the animation going, and you want it to stop growing/shrinking at a certain point, right? Your new best friend: the Stop parameter behavior. Stop freezes a parameter in place. Simply add a Stop to the Position parameter of the object, having the Stop begin at the last frame of the Grow/Shrink. To add the Stop, just right-click on the Position parameter name in the Inspector, and select it from the list. Stretch the Stop’s length out for as long as you want the scale to hold.
Hope this helps. Good luck!