Some soil will resist growth, but when a persons heart is right & receptive. The harvest God can produce in and through them is beyond calculation. The point that he's making is that the fruitfulness that God desires for his kingdom and for his people is beyond human comprehension or ability.
In fact, one reason that Jesus told this story is to encourage his followers, because we, you and I, are not just soil. We are also called to be sowers. We're given the privilege of sowing seed – sharing the hope, love and grace of the good news of Jesus. Part of what he's saying here is just know this ahead of time, when you sow seed, sometimes it will fall on non-responsive soil. Often it'll fall where the soil is hard or where it's shallow, and it'll look good for a little bit and then it'll disappear. Or where there's clutter, and it'll spring up but then get choked out.
Jesus says often this will happen as you're sowing seed, and you'll be tempted to give up. But Don't Quit!
Because the harvest is plentiful – the harvest is coming… sometimes conditions are right, and when that happens, watch out. After all – it’s God who does the growing; growth is mysterious in some ways and can’t always be predicted.