We would need details for each movie to know that. Julia still makes movies and she's still with the same studio, so both are probably happy with the sales. Julia has the busty & slender look that appeal to a broader potential audience. Ichika is fitting into the loli category. Lolis have never been very popular in JAV, it's a niche. The exception might be Tsubomi, but she is a special case. Also, smaller studios and being freelance reduce the chance of getting big promos for your titles by the studios (as you aren't exclusive). Abe Mikako never got mainstream in the JAV world. Only people interested in lolis know her. The look is also one big factor. For example, Kawakita Saika is ranking #3 in DMM physical sales, and she has max 10 movies.
I hope you aren't talking about breast implants because it's getting really annoying read the same comments over and over while there are replies about it on the forum. She might have done some stuff to her face, even if a small weight loss and makeup can change a face completely (her makeup is different for sure).