Yeah I’m not buying this.
Ishiyama-sensei was an assistant to Eiichiro Oda, so it’s fair to assume Hanagami was based on him.
In One Piece, for the main cast alone Luffy, Chopper, Franky, Brook, and Jimbei all have wildly different silhouettes that make them easily recognizable. But Nami and Robin have virtually identical silhouettes that make them look both just like… two girls with big boobs.
Hell, this is for many series as a whole. For male characters, you could have a variety of wild body shapes and types. In Hero Academia, Midoriya and Mineta and Koda and Shouji all look wildly different. But the girls all generally have a similar body type, so adding big boobs wouldn’t change a thing.
If you like adding boobs to your series because you like drawing them, then sure. All the more power to you. But don’t try to twist it as something that makes artistic sense to do.