In the studies on the time dependence of size distribution of aggregates, generally, the volume of aggregate is used as internal coordinate, and the similarity variables based on the volume are used for the analysis of self-preserving size spectrum. However, it is one of the most important characteristics of aggregate that the porosity of aggregate changes through the process of aggregate growth.Hence, in the present paper, seven basic models were proposed concerning the manner of the porosity change, and the most general internal coordinate was determined and the corresponding similarity variables were drawn. Namely, it was made clear that the volume as the internal coordinate and the corresponding similarity variables are usable only in the unchangeable case of the total volume of aggregates in the system, on the other hand, the most general internal coordinate is the weight of aggregate constituents. And also, the similarity variables based on the weight were drawn.