In MongoDB, the Subset Pattern is used to reduce the size of the working set by only bringing into memory a subset of the overall data. This is done by splitting large documents into smaller ones based on the most frequently accessed data. The idea is to store the most used part of the document in one collection and less frequently used data in another, allowing for shorter disk access times and reduced pressure on RAM capacities. By using this pattern, developers can improve performance and scalability of their applications.