coin cell lithium rechargable batteries are the most space-efficient I have found and they enable totally new shapes of portable electronics as I see it compared to the "foil bag" batteries - which comes with 2-3 mm long ends that is the foil that is folded I guess to protect the load inside.
coin cells are about 30% smaller than their counterpart "bags". not counting the flaps.
For you in your case of a smart watch I assume you want a thin wide battery. German Varta has 2 new coin cells available at 50 & 100 mah, 12 & 16 mm wide in diameter eespectively, and 5.6 mm tall. http://www.varta-microbattery.com/appli ... S63125.pdf
As u can hear On my terminology I'm not a battery commodity guy. I'm a designer in portable electronics and new shapes of batteries defines the ID in my field as I see things, and theVarta is hot new stuff. just rsmping up production now...
hope this helped you.