Gangut is the smallest BB in the game, shorter than all of the heavy cruisers and only marginly longer than the Aganos (or RJ by a mere meter) so her as the short BB kind of makes sense. Also she's a Yadokari girl who are always shorter
She apparently has the smallest boobs of any battleship in the game as well.
Sara is never really depicted as tall despite the massive size of the ships, I think this is more just a shot at Gangut being short
Comparative height is of course fairly inconsistent among artists, but I've seen her portrayed as considerably tall or seen her height remarked on by others quite a bit, and I can't immediately recall her having been drawn as more then marginally shorter then anyone but the most consistently "giant" girls. Mostly it seems to be a matter of if the author is aware how large the actual ship was, most authors are aware that Iowa was the longest battleship ever built so she's fairly consistently portrayed as among the tallest if not the tallest. They seem less aware that Saratoga was actually fractionally larger and the longest warship period in the world during the actual war.
Those that are and tend to try and bring such ship based details into their 'canon' seem to fairly consistently portray her as quite tall in a similar way to Iowa though.