You didn't need to "assume". You could have looked up the case sizes easily. There's a "Size and Weight" section on each watch's web page. For example:No way. The difference between case size is more than 1/20th of an inch on each side. Or at least I assume it is. It would be nice if the cases fit together due to the center pin that you point out. But I doubt the difference is that subtle. ...
http://www.apple.com/watch/apple-watch-sport/silver-aluminum-case-white-sport-band/
The width is 33.3 mm or 35.9 mm. The difference is 2.6 mm, or just over 1/10 of an inch. So a wide band will stick out 1/20 of an inch on each side of a narrow watch. Which is exactly what kdarling wrote.