I offer patient's a simple analogy to illustrate what we mean:
Q: If you take a baseball and you cover it with a silksheet, what will it look like?
A: Like a baseball, you will even see the stitching.
Q: Now if you take that same baseball and cover it with a comforter what will you see?
A: It depends on how thick the comforter is but generally you will see a softly shaped lump.
So now the question is what do you have on your chest, silk sheets or comforters?
The way we determine this is by looking at your chest, if I can see your ribs (under the muscle) then you anatomy will have a hard time disguising the implant whether it is placed under or over the muscle and you are more likely better off with a silicone implant. Another way of measuring this is to pinch your upper chest skin (similar to pinch and inch of your waist on the Special K commercial). If you have less than an 2 cm (under 4/5 of an inch) than you have "silk sheets"
If you have a thicker skin/fat/breast layer that pinches over an inch than you have the option of going under or over.
Now the exception to the rule:
Q: What happens if you change the baseball to a volleyball?
A: All bets are off, whether you put it under or over the muscle it will not look natural.
The last issue is the implant shape. There are some anatomic shaped implants but I have generally found them to produce no significant difference in outcome with the exception of breast cancer reconstruction. However, this is my personal opinion.