It's not just because they are firm but it's something else that i can't really explain.
Miho Ichiki/Anna Okina is another example of boobs you can tell are fake just by looking at them.
Ruri Saijo, Anri Okita and Julia all have very firm boobs for their size but they still look natural.
Man I LOVE Miho Ichiki's (Anna Okina) obviously fake looking boobs. Same with Sena Sakura.