<ThatGuy>
Samsung introduced 3G connectivity in the Samsung Gear S2 already two years ago, and added 4G LTE in the Gear S3 last year.
</ThatGuy>
That size/battery life stuff is what took all the work, and why Apple waited until it was able to ship without the compromises.
The size increase in Gear S3 was more dictated by fashion than necessity -- after the Gear S2, Samsung had market data that their primary buyer group favors larger watches in general, so they decided to bulk it up and increase battery life. (A lot of men actually like that "yo-yo on the wrist" look, for reasons I don't entirely fathom. I guess it's sporty.)