^ couldn't have said it better myself.
it's all about outfits, angles, positions, etc. the best examples i could find are in Nacchi's ecru: in the mud scenes, her breasts are pushed up and look HUGE. compare those to the shots where she only has a bikini on. they're considerably smaller-looking, thanks to lack of padding, and poses like the ones with her arms raised.
also, i see nothing about Rika's breasts that look fake.