Sorry, there's not much we can do about this: when you change the size of the object the engine has to destroy the physics body and create a new one in its place at the new size, and since the impulse was applied to the old object it gets lost. You don't need the physics behavior for the specific example in the .capx, but if you really do, try to use an object which stays the same size (or at least changes in single step jumps), and then have an object on top of it that gradually changes, so it has the same appearance.