Forgiven a fortune, he would not forgive a trifle.
It is also called the parable of the unmerciful servant. Peter had just asked how many times he should forgive, and Jesus had answered "seventy times seven". This story follows, as the explanation.
It treats forgiveness as something received before it is given — and shows what happens when someone keeps what he was given.
Shouldn't you also have had mercy on your fellow servant, even as I had mercy on you?
The disproportion drives the story. Ten thousand talents was an absurd sum — a single talent was many years of a labourer's wages. A hundred denarii was about a hundred days' work: real money, but payable. The servant walks out of an impossible forgiveness and cannot pass on a small one.
The king does not punish him for being hard on debtors in general. He punishes him for having received mercy and not passing it on. It is the same link that runs through the Lord's Prayer — "forgive us our debts, as we also forgive our debtors" — told as a story.
Which is why it answers Peter's question better than the number does. Anyone still counting how many times they have forgiven has forgotten the size of the bill that was forgiven them. "Seventy times seven" is not a higher target; it is what happens once you stop looking at the other person's debt.
Walking with Jesus has a collection called The Parables of Jesus that takes them one a day — each with the question he was answering, a short reflection, a prayer, and one thing to carry into the day.
It is a one-time purchase, about five minutes a morning, and it never asks you to catch up on days you missed.