addendum:
I have to ammend this. I thought about it today, and it was another job that we used 4x8 cylinders for, not the augercast pile job. I've been on two of these, both times I used 2x2 cubes. There are ASTMs governing the sampling of grouts, but to my knowledge there's no table spelling out maximum aggregate size corresponding to sample size. Sorry about your predicament. Hope that helps, a bit.
I'm not sure how you would reason with a PE that requires historical testing for consistency, given that you would have to perform trials and such, not timely or cost effective. I would just say that given the aggregate size, namely sand, 2x2s are more economical than 6x12s. ASTM C31 spells out mazimum size aggregate v. sample container and the trend goes larger with larger MSA, and smaller with smaller MSA. It seems also that you would get different results from a 1/1 ratio break than with a 1/2 ratio break. That and I've never heard of anyone making 6x12s out of grout. That's just not how it's done.
That's the best I can do! Good luck.