Technically? It’s easy when you have more space, larger batteries and higher thermal limits to have more features. But the original iPad had technically inferior processor (256MB RAM vs 512MB than the iPhone that came out 3 months later).
The Watch being smaller, it’s technically much more impressive that it has an always on display, cellular, WiFi, Bluetooth, GPS, heart rate sensors, altimeter, gyroscope, 32GB Storage, and it still gets 12-16 hours of battery life.
While the Apple’s A series is already 2 or 3 years ahead of everything else in the mobile market, nothing is even coming that close to the S series chip.