Sandgoose while your argument is true at face value, I think it's basically meaningless.
With 48 games under your belt you can't make a "low sample size" argument for why he's up on you. You've had enough wins and losses to establish your position on the ladder
GR is very good at yielding an accurate rating regardless of how many games you play once you've reached that "natural rating" where you really belong (which only god knows). After that point, it doesn't matter how many games you play, since your new rating is based on most recent result and old rating only.
In fact, if I remember the math right, SD has a steeper slope than you do, because he's more heavily penalized for a loss. If he wins 51/100 and you win 5/10 (all else being equal) then you'll actually gain on him in GR, despite his "superior play."
Basically, instead of "People who play more games raise faster in GR if they win," you could have said, "people who win a higher percentage of games raise faster in GR," The two statements would be equal in meaning, but the former implies that games player per unit time is a factor, when it isn't.