I think you could do it with an instance variable. So set up a variable called var_Scale, set it to 1 to start with. When there is a collision you say:
> Subtract 0.25 from var_Scale
> Set Scale of Sprite to 1
> Set Scale of Sprite to (var_Scale)
That way each collision should reduce by 25% of the original size right?
Edit: I tested and this worked, although there might be a more elegant method