Yes u might be right on diameter

12.9mm With .5 of that being a nickel coating! But length?
yeah it gets to be a toss up...that's why the term smallest is debateable...are we talking diameter or are we talking length...
we're also talking about all mechanical versus wired too...
not that I have a dog in this hunt...since the 10440 battery just isn't my thing...
just that the "smallest" designation is somewhat misleading...there are so many other factors depending on what the person is looking for...
For me personally the precious is a non-contender...when you balance size versus all mechanical, where it's made (so where it'd have to go for repair), style of switch, materials, cost of unit, etc.