See Ashley's response here to a similar problem reported as a bug. Every time you change animation frame, or resize an object (as in your case), the physics body inside the physics engine has to be replaced.
[quote:2yka3vxj]The Physics behavior treats a change of frame as destroying the physics body and teleporting a new one in its place (to take in to account changing collision poly, size, origin etc). Therefore animating objects keeps destroying the physics body you're applying force to.