I'm doing some substantial infantry work right now. Struggling with the differences in scale between infantry sculpts, ba sculpts and their transports. Vehicles are typically too small anyway in comparison, and the newer (better) infantry is a little larger. I typically will use the smallest (oldest) infantry if I'm putting it on a base with a mech or vehicle. But if it's a base of infantry alone, you can't beat the new sculpts for detail.
So, they're all representational, I guess. :) Although I might experiment with a DA plastic vehicle and see how that looks, scale wise, compared to the rest. I know Worktroll uses that method to good (and prolific!)effect.