How can the term sonata apply to a string quartet or symphony?

The term "sonata" can be applied to a string quartet or a symphony because these works are based on the same principles of musical form and development that were established for smaller instrumental groups during the mid-18th century.12

While the term "sonata" is most frequently used for solo or duo works, it functions as a broader designation for multimovement instrumental compositions that share a common structural foundation. Specifically, these larger works follow the sonata's division into several movements—typically four in the Classical tradition—and utilize the sonata form as a structural basis for their first movements.1234

In this context, a symphony is effectively considered a sonata for orchestra, and a string quartet is a sonata for four string instruments. Although internal differences exist—such as the symphony's often larger scale or the concerto's dual-exposition structure—the majority of these larger instrumental forms are closely related in their total organization, relying on the interplay of thematic development and key contrast that defines the sonata principle.254