Fuel Lines-Why is the return line smaller?

[BBShark]

What is the reason for making the return line smaller diameter than the supply? I am using a 3/8 supply line and see that most manufactures use a 5/16 return. Well, I can't find much in the way of -5 AN fittings (5/16) and would like to use -6 AN (3/8) supply and -6 AN return.

Am I missing something? Any reason why this will not work?