Mathematical modelling is endorsed as both a means and an end to learning mathematics. Despite its utility and inclusion as a curricular objective, one of many questions remaining about learners' modelling regards how modelers choose relevant situational attributes and express mathematical relationships in terms of them. Research on quantitative reasoning has informed the field on how individuals quantify attributes and conceive of covariational relationships among them. However, this research has not often attended to modelers' mathematization in open modelling tasks, an endeavor that invites further attention to theoretical and methodological details. To this end, we offer a synthesis of existing theories to present a cognitive account of mathematical model construction through a quantity-oriented lens. Second, we use empirical data to illustrate why it is productive for theories of modelling to attend to and account for students' quantitative reasoning while modelling. Finally, we identify remaining challenges to coordinating different theoretical accounts of model construction.