What does Scrooge mean by saying that they should "decrease the surplus?"
This statement reveals a couple of things about Scrooge. First, it reveals his cold-heart; he would rather see the poor people die instead of helping them, and by dying, they would be doing a service to the world by "decreas[ing]the surplus population;" You must keep in mind that when Dickens wrote the Carol, London, England was inundated with people looking for work; this was due to industrialization where machines were replacing people who worked on farms, so as they were replaced by machines, they began to flock to the cities looking for work. Thus the city was over populated, which caused many problems and became a huge financial burden.
Secondly, it reveals that Scrooge is a follower of Thomas Malthus who in 1803 published an essay titled "Essay on the Principles of Population." His essay posed the argument that
"A man who is born into a world possessed, if he cannot get subsistence from his parents, on which he has a just demand, and if society do not want his labour, has no claim of right of the smallest portion of food, and in fact, has no business to be where he is. At Nature's mighty feast there is no vacant cover for him. She tells him to be gone . . ."
Men like Malthus and Scrooge believed that war, famine and pestilence were necessary in order to keep the population down, especially the poor people.
What he means by this is pretty nasty -- he means that the poor people should just go off and die.
'If they would rather die,' said Scrooge, 'they had better do it, and decrease the surplus population.
He says this in the first stave of the story. In this episode, the two men come to Scrooge's office to ask him to give to charity. They tell him that there are all these people who are in need during the Christmas season.
Scrooge tells them that he already helps pay for the workhouses and the poor should go there. The men tell him that some of the poor would rather die. At that point, Scrooge essentially tells them "let them go ahead and died."