In my opinion, Edwards is trying to point out how easy it is for humans to neglect details that are small in our world. For example, you can move a web out of your way when walking and not even think about the fact that you just destroyed a spider's home and work of hours and days. Humans step on insects unconsciously every day. He juxtaposes this idea next to the God's capacity to recognize his enemies in the world. God is big. His enemies are small. They are plentiful. To start discarding his enemies and to cast them to hell would be simple for God, just as simple as it is for man to step on a worm or push a spider web out of the way. Edwards' purpose is to demonstrate our possible insignificance to God. With billions of people, God could find people worhtless and unnecessary to maintain their lives.
However, as the sermon continues, Edwards uses this hyperbolic demonstration of God's size in comparison to man's in order to eventually call man to believe in God with great faith.