Schemas are groupings of tables and other entities that can be defined within a database. You can think of them like of a namespace in programming languages. You can have the same table definition, within the same database, defined multiple times (each in a different schema) and each holding different data.

By large and small we are referring to the amount of data each schema holds currently. They can grow over time and some of them may become very big while others will remain small (storage wise).