Colligative properties are defined as the "properties of solutions that depend on the ratio of the number of solute particles to the number of solvent molecules in a solution, and not on the nature of the chemical species present (wikipedia)."
When a solute is dissolved in a solvent, the number of solvent molecules near the surface decreases, and the vapor pressure of the solvent decreases. That's why boiling-point elevation, freezing-point depression etc correspondingly occurs.
If the reason of elevated boiling points is due to decreased surface area of volatile solvent molecules, then why does the size of the solute not matter? Won't larger solutes occupy more space in the solution?