Colligative properties and sizes of particles

Why do colligative properties only depend on the number of particles and not the size?


\*My argument being that since the root cause of these properties is the occurrence of solute particles on the surface, reducing the vapour pressure of the solvent, shouldn't bigger molecules, such as urea, give a much more appreciable deviation in properties, as compared to smaller ones like NaF?




[Colligative properties](https://en.wikipedia.org/wiki/Colligative_properties) From Wikipedia



> 
> In chemistry, colligative properties are 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. The number ratio can be related to the various units for concentration of solutions. ***The assumption that solution properties are independent of the nature of solute particles is only exact for ideal solutions***, and is approximate for dilute real solutions. In other words, colligative properties are a set of solution properties that can be reasonably approximated by assuming that the solution is ideal.
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> 
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Emphasis in above text is mine.


So it the **assumption** that colligative properties are independent of particle size, not the reality. The assumption is very useful, but not perfect. 


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Vapor pressure reduction is not truly explained as a mass transport phenomena where solute particles at the surface of the solvent prevent solvent molecules from entering the vapor phase. Since vapor pressure depression is dependent on solute concentration, which is uniform throughout the solution, the proper explanation is the increased entropy of the solution as compared to the pure solvent. The entropy increase is not dependent on the size of the solute, but the relative quantity of solute to solvent molecules (expressed as the concentration of the solute).


<http://www1.lsbu.ac.uk/water/colligative_properties.html>


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