Currently a 30 DD. 4'11" and 92 pounds. - What is the largest implant size I can get without back pain?

I wear a 30DD bra, one cup fits but the other is slightly loose. I measure about 26.5 at the band and 29.5 around the breast. The difference in size doesn't really bother me since it is so common. I am 4’11” and 92 pounds. I don’t feel “flat” but it would be nice to have a bigger cup size and more projection, especially in non-form fitting tops. I have also lost weight so my cup size has decreased, and in both tight and loose tops it would be nice to have a bigger size. Ideally I would like to get a breast fat transfer, but after calling a local plastic surgery office the receptionist said it may only increase the size by .5 cups. My BMI is under 18.5, which they said doesn’t matter but I’m not sure if the small area of fat around by abdomen will be enough for more than .5 cups. I am open to either high or extra high profile saline or silicone implants. I work out 3-6 days a week, which usually involves weights and running. I would like to increase the cup size but I am wondering what size starts to cause back pain or pain during exercise. I am also wondering about breast implant illness. Does the human body in general always react negatively to implants, and the patients who develop BII have the worst reaction? Or is BII a reaction that occurs in only certain people, like how only certain people are allergic to nuts?