Well, we do know that Apple stated that it should last around 19 hours...and 19 hours is the worst battery time for a smart watch (based on reviews I've read).
First, you do realize that there are TWO watches, ones about 33% in volume smaller than the other.... That most of the extra volume in the bigger one will undoubtedly filled with battery (since functionality is the same for each watches). So, how long does that bigger watch last hmmm. Quite a bit longer (more than 33%) no matter what the baseline is for the smaller one.
Secondly, Apple has issued NO SUCH STATEMENT (no 19h with various breakdown of usage that you seem to be referring too). Cook said lasts a day under heavy usage; that's the only real info we got. We can assume that this applies to the smaller watch first since it is the one with less battery.
How much usage is heavy usage makes all the difference in the world in how you compare the watches. So, comparing right now to other watches is plain laughable. What are you comparing exactly?
I've actually handled most of the current smart watches and you can kill the battery on any of them in less than half a day, no sweat, if you want to. But, what does it actually prove? I can also do the same to a smart phone. Most of those other watches were functional (if very clunky to use), but were esthetically horrible, just horrible. I would never in a million year want to wear any of them. If apple can solve this issue, they'll be way ahead of the Android crowd no matter what they do.
Finally, the biggest Apple watch is 10% smaller than the average Android watch and smallest one is 40% smaller! That should tell you exactly why, especially for the smallest one, battery optimization is a challenge and getting a day out of that small watch is in fact a feat.