All my cats are suddenly huge

Hello all,

We have three cats (7yrs, 13yrs, 18yrs) that recently started eating
more than usual.  All three have grown huge in less than three weeks
-- far more than the extra food should allow and all at the same time.
 They are not just fatter -- they are taller, longer and bigger-boned.
 I have no idea what is going on.  They previously had been thin cats,
the oldest even being ill due to her age, but she now looks like a cat
ten years her junior and is bigger than she has ever been.  Does
anyone have any ideas what is going on??  I feel like I am in an
episode of the Twilight Zone or maybe Pet Cemetary :-)  Could some
neighborhood oddball be slipping them growth hormones??

Sarah and Chris