We analyze size-scaling effects on the functional form of adsorption isotherms in microporous materials. In particular, Keller adsorption isotherms are carefully discussed, focusing attention on the description of energetic/geometric heterogeneity and on thermodynamic consistency (low-pressure behavior). In order to justify the Freundlich behavior shown by the Keller isotherms, a Monte Carlo model of preferential adsorption is discussed. Despite its simplicity, this model exhibits extremely rich behavior.