Britannica Sources used in this answer

Why can changes in a quasar's brightness reveal its small size?

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six quasar host galaxies — Quasar Six quasar host galaxies, as observed by the Hubble Space Telescope.
Shown are apparently normal, solitary galaxies (left), colliding galaxies (centre), and merging galaxies (right).

The observation that a quasar can change its brightness significantly over a short period—such as a few days—provides a direct constraint on its physical size.

Because nothing can travel faster than the speed of light, a change in brightness across the entire object cannot occur faster than the time it takes for light to travel across it. If a quasar’s luminosity fluctuates on a timescale of a few days, the region producing that light cannot be larger than a few light-days across. This indicates that the energy-emitting region is extremely compact, roughly the size of our solar system, despite the object's immense luminosity.1