Why do large molecules leave a gel chromatography column before small molecules?

In gel chromatography, also known as size-exclusion chromatography, the separation of molecules is based on their physical size as they pass through a porous, semisolid stationary phase.12

The stationary phase consists of porous particles that contain channels or pores of specific sizes. The mechanism works as follows:

• Small molecules: These molecules are small enough to diffuse into the interior of the porous particles. By entering these pores, their path through the column is effectively lengthened, which restricts their flow and slows their progress.12
• Large molecules: These molecules are too large to enter the pores of the stationary phase. Consequently, they are excluded from the interior of the particles and remain in the mobile phase, flowing through the interstitial spaces between the particles unhindered.12

Because large molecules are excluded from the pores and travel only through the spaces between particles, they move through the column more rapidly and elute (leave the column) before the smaller molecules, which are delayed by their diffusion into the pores.12