You hands are different so change your grip
Everyone has something slightly different about their hands and this has been know to firearm manufacturers for some time. Many military issued firearms were not very good for people that were below or above average sizes and that has been the norm for a very long time. Not until the accessorized AR15 have there been so many opportunities to improve a firearm to an operators standards. To each his own when it comes to which handgun fits your hand, but a good fit can be better if you can use Hogue grips or Magpul grips.
Magpul is king of the AR15 accessory world but Hogue, Packhmayr, Decal and Ergo cover everything else. If you have to carry a handgun or rifle all year long and live in an area where the seasons change magpul accessories will probable get you covered. There is not always size variations, but angle variations can mean something too. There are big differences between the grip angles of a Smith & Wesson M&P, Glock 19 and a Springfield 1911 so not all rifle grips for your AR15 are the same.