You're missing the point. The problem isn't that "plump" manga aren't being tagged as BBW, it's that the rules force them to. Let me put it like this; say I wanted to find something with small breasts, I would use "pettanko". If I wanted to find something with exaggeratedly giant breasts, I would use "huge breasts". But if I wanted to find big breast without the ones on the far end of the size spectrum, I would use "large breast" and exclude "huge breasts".
Plump is a subset of BBW meaning that its only function is to exclude plump from the rest of the BBW tags. You can't use it to find manga with characters that of only plump but not on the larger end of BBW.
I don't get it. I fail to understand what you mean, I'm really sorry.
Here, let me explain the system:
Searching BBW -Plump (or just -Plump) = BBW girls that arent plump, in other words, any other BBW that doesn't fall in the category 'plump' (a.k.a really big girls).
Searching Plump = ONLY books that have plump girls.
Searching BBW only = all of them.
BBW is working as a super category, that should be used for general searches. While its children category "Plump", is being used as to either show a small portion of the super, or to see the rest of the super. this way, any plump girl is (by default) also a BBW girl, while you can still find only them if you want..
Same happens to Large Breasts and Huge breasts. By default, any Huge breast is also Large, so huge breasts are always tagged as Large as well. As you said, if you want to find large breasts only, you just exclude huge from the search. If you want to find only huge ones, just type search for huge and it will show only the huge ones.
In other words, it's the same thing here: Large breats is the super category, while huge are its children.