How did Romantic symphonies differ from Classical symphonies in form and length?

During the 19th-century Romantic period, symphonies underwent significant transformations in both length and structural approach compared to the Classical period (c. 1740–1820).1

Changes in Length and Form

• Increased Scale: Romantic symphonies generally grew longer and more expansive than their Classical predecessors. By the late 19th century, the scale of these works—and the orchestras required to perform them—had grown to such an extent that it eventually prompted a reaction toward the more disciplined, compact forms of the 20th-century Neoclassical movement.1
• Structural Flexibility: While Classical symphonies typically adhered to a standardized four-movement structure, Romantic composers often utilized looser forms. Some composers prioritized emotional expression and programmatic content over strict formal discipline, occasionally leading to a departure from the traditional sonata-allegro form.231
• Unification Techniques: To manage the challenges of larger-scale works, Romantic composers developed new methods to unify movements. These included the use of extramusical programs, recurring themes such as the idée fixe, and thematic transformation, which helped maintain musical coherence across extended compositions.415