Instance variables of each ball:
sizex <- width
sizey <- height
startx <- sprite.x at start
starty <- sprite.y at start
You probably know the center of your rhomb in x and y too. So the movement and size can be tweened quite simply back and forth. There are behaviors that do it, like LiteTween (behavior-litetween_t70700) or you can do it by yourself with the event system and some simple expressions (http://gizma.com/easing/ for reference).
About balls touching each other:
Be aware that they will necessarily touch each other, if they all reach the center of the rhomb, so you might want to stop a tiny bit earlier.
You can try to tweak the size reduction yourself (let it reach "0" a bit earlier than finishing the movement), or do the math and check how large (relative to starting size) each ball can be along the timeline.
I'm not sure how moving the balls via events by setting the position interacts with the physics behavior. Maybe there are problems or even easier solutions I am not aware off - I have not used the physics behavior so far.