Why does the Ghost of Christmas Present say that if Tiny Tim dies he will decrease the population, in A Christmas Carol?
When the Ghost of Christmas Present says that the death of Tiny Tim would decrease the population, he is not expressing his own sentiment. The ghost is, in fact, repeating something that Scrooge says earlier in the book. You can find this in the opening chapter, when two gentlemen call on Scrooge to ask for a charitable donation. Scrooge refuses to donate any money and instead asks the gentlemen about the workhouses:
"Many can't go there; and many would rather die."
"If they would rather die," said Scrooge, "they had better do it, and decrease the surplus population."
By repeating this words to Scrooge in chapter three, the ghost is forcing him to confront the opinions that he once held about the poor. In the first chapter, we learn that Scrooge has no charitable feelings: he believes that poor people are like a plague on society who have fallen on hard times because of their own doing. The workhouse and the prison were places where the poor belonged, far away from Scrooge's world of money and success. If a poor person died, it would simply reduce the burden on the existing population.
By the time that Scrooge is visited by the Ghost of Christmas Present, however, he is much changed in this opinion. He sees poor children, like Tiny Tim, as innocent victims of circumstance and this is evidence of Scrooge's transformation in the book.