A well-fed Galactus wields cosmic power beyond what even Superman can match.
Superman is one of fiction's most powerful heroes, but Galactus operates on a fundamentally different scale. The Devourer of Worlds is a cosmic entity who predates the current universe and wields the Power Cosmic, an energy source that can reshape reality itself. A fully fed Galactus is considered one of the most powerful beings in all of Marvel.
Superman's incredible speed, strength, and heat vision are formidable against most opponents, but Galactus can manipulate matter and energy at a molecular level, create impenetrable force fields, and project energy blasts that can destroy planets. Superman has survived planet-busting attacks, but Galactus operates at star-system and galaxy-level scales.
The variable is Galactus's hunger level. A starving Galactus has been challenged and even defeated by teams of heroes. If Superman catches Galactus at his weakest, he could potentially win. But a well-fed Galactus would simply transmute Superman into something harmless.
Galactus wields the Power Cosmic, can manipulate matter at a molecular level, and has destroyed entire star systems. His power at full strength dwarfs anything Superman has faced, and he can attack on dimensions Superman has no defense against.
Superman has near-limitless strength under a yellow sun, can fly at faster-than-light speeds, and has survived attacks from cosmic-level beings. His heat vision and freeze breath give him ranged options, and sun-dipping could massively boost his power.
A well-fed Galactus wins this more often than not through sheer cosmic power that transcends Superman's physical abilities. A starving Galactus makes this much closer, but at full power, the Devourer of Worlds lives up to his name.
Galactus also fights
Superman also fights