Small Business vs Large Business IAW FAR 25

I'm looking for direction on the rationale for some numbers given in FAR 25. FAR 25 states that when a Foreign Concern (FC) submits an offer there are certain requirements for evaluating their price when compared to domestic firms. The Buy American Act gives benefits to domestic concerns when competing against an FC. If the leading domestic concern is a Large Business (LB) they are given a 6% advantage. If the leading domestic firm is a Small Business (SB) they are given a 12% advantage over the FC. In other words an FC must be 6% or 12% cheaper depending on the clasification of the domestic firm.

My question is, where did they come up with the 6% difference between a SB and a LB? I'm no longer looking at comparing them to an FC, but how they relate to each other. Was a study done that showed LB's are generally 6% cheaper than a SB? I mean why 6%? Why not 4 or 5% OR 7or 8%? Can you direct me to some sort of documentation that explains this?

Thank you

Gregg