The 3G model of the Gear S2 is roughly the same size as the Apple Watch Series 2/3 42mm model.
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.)
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.)