The kanji used for "hinnyuu" 's hin is not 牝 , it's 貧. This character means impoverished, destitute, or bankrupt. It's the bin in 貧乏 (binbou), a word you all probably know which means "poor."
So quite the contrary, hinnyuu is not a kind appellate and it does not mean "average breasts" or "womanly breasts." It's an insulting term, the sort of thing you'd never hear a DFC'er say.
Which is why you will hear me say hinnyuu often and with no reservation. ;P (Kyonyuu for life! "Small Boobs are No Boobs" I say!)
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As for bakunyuu, Gakido's got it covered. Just like he said, the idea is (in my own words) "boobs so big you think to yourself 'THEY'RE GONNA BLOW!'", as if they were overinflated balloons or water-balloons just seconds away from bursting.
So to me, I'd say that the breasts in Inyouchuu Shoku attached to that one girl with the glasses and the dark-colored hair would be bakunyuu, whereas Reia's breasts (the girl in Rein's current avatar, the main character in the Ikusa Otome Valkyrie series) would be kyonyuu. She's got some nice big boobs that could smack a guy in the face and give him a bloody nose, but they're not so big that they strike you as unreal.
General rule of thumb,
kyonyuu : DD's in American bra cup sizes :: bakunyuu : two cup sizes beyond that
Like, even though her breasts are allegedly real, I'd say Chloe Vevrier is a pretty good example of a real-life "bakunyuu." Her breasts are just unbelievably large.