I figure that even inbred OP varieties have a fair amount of genetic variety. They are closer than fraternal twins, but not as close as identical twins. (It's just my opinion, I can't prove that.)
I bet if we were starting 100 seeds, 5-10% would have started as fast as your biggest plant.
Selecting for size at this early an age might be a bad strategy, because you might pick all the plants that are good at shooting up fast for a few weeks, but lack genes needed for sustained growth, or high yield of fruit, or who-knows-what.
Again, just my theory.