If your pectoral muscle are compacted (not sure if that’s the word!?), i.e. if you have super strong pectoral muscles through exercise, then they may not be able to do under the muscle (partials).
I know this as my sis in law had to have overs, even though she was a 30aa basically totally flat chested. She had no breast tissue at all so her only choice was to have small implants, 250cc hp overs.
Hope this helps xx