Yeah, Tween behavior really lacks looping and reversing options. I posted this idea a long time ago:
You can simply run the second tween after a delay: Tween "increase_size", wait 0.2s, Stop Tween "increase_size", Tween "decrease_size"
Or I often use this simple effect:
On Clicked Button
Button set scale to 1.05
Button set scale to 1