Under or over the Muscle?

My operation date is January 24th and I’m still debating whether to go under or over the muscle. I’m about an A cup now and wanting to go to a C, not really much bigger. I’m 5 ft 5 and 124 lbs. I don’t want them to look stuck on and I’m wanting a really natural result. The surgeon has suggested 275 cc over the muscle or 300 under. I worry that over the muscle will look too round? Or is that not true? Apparently they move more like natural breasts over the muscle? Any help and advice welcomed. Thank you :)