I'll look back through some things, but the biggest problem with oversized convexity is in the potential for stress risers and overlap welds in a fillet weld as the amount of convexity increases.  Thus, it is really correct to gauge it either way as to the geometry but the codes just picked one when originally written.  They are related, the larger the leg, the wider the face.

While one could make an argument that most all other dimensioning for fillets is done by leg length then the covexity should be determined by the same factor.  But that may just be one area that has never been addressed to get it changed by and of the D1 committees.  Getting things changed is not an easy thing to do.  But it doesn't really matter that much which one you use.

Have a Great Day,  Brent