The CR2032 battery alone is about double the thickness of a credit card. And that's before the circuit board which is probably a little thicker than the battery. The final project will still be a bit larger than a credit card, but you might be able to make it smaller than it is now. Assuming you have a way to cover the battery and are willing to give up the buttons to lock/unlock the doors. You'd have to crack open the plastic key case with a little Dremel tool or something, hack apart the current circuit board to remove the buttons, and mount the whole thing in something smaller. I certainly wouldn't be brave enough to try it.