Yes the batteries are small, but other brands last much longer than Apple watches
No they don’t. Apple Watches have very large batteries and very efficient processors.
Other smart watches only get longer battery life if you avoid power hungry features… and you can do that with an Apple Watch. Apple’s largest watch will last a full 30 days if you don’t use power hungry features like wifi, heart rate monitoring, music playback, Siri, GPS, etc etc. The smaller Apple watches will last almost as long in the same mode.
Most smart watches that advertise long battery life have all (or at least most) of those features, and they don’t last long at all if you take advantage of them. The Garmin FR965 (their largest watch) advertises “up to 25 days” battery life but that’s with everything disabled (and it’s also less than the 30 days you get with an Apple Watch). If you read the spec sheet for the Garmin FR965 it advertises “Up to 8.5 hours” if you make extensive use of all of the available smartwatch features. And that’s the biggest/heaviest Garmin. The popular Garmins have smaller batteries - like half the size of the battery in that Garmin.