You are assuming that the smaller pebbles do exist, so it is the pebbles is forcing the outcome and not the sieve.
That makes absolutely no sense. If we get an assortment of randomly sized pebbles, it makes sense that some would fit through a sieve and others would not. The pebbles are not "forcing" anything. It's the sieve that sorts one set of pebbles from another.
The outcome will naturally happen but how the small pebbles came to existence is the issue.
In the case of pebbles, erosion. In the case of living organisms, mutation. In either case, the cause the variation is irrelevant. What matters is that there is a process which selects and separates them.
The DNA complexity isn't that simple to compare it to pebbles and sieve.
I'm not comparing the complexity of DNA to anything. I'm comparing the process of natural selection selecting from a random assortment of living organisms to a sieve selecting from a random assortment of sizes of pebbles. It really is that simple.
Do you or do you not understand that when you sieve something, just as when you apply any kind of selective pressure to any random assortment of things, what you end up with is more specified than the whole? This is very simple logic.