The size-weight illusion refers to the commonly observed inverse relationship between physical volume and perceived heaviness. The present research proposed a new way of classifying the various types of models and examined the validity of each type through two experiments on haptic touch with and without vision. A feature-complete factorial design was used, whereby the stimuli varied factorially in levels of volume and mass, and participants gave perceptual reports on each dimension. The data were analyzed through tests of response proportions and multidimensional signal detection analyses. When visual input was eliminated, results indicated a lack of both perceptual and decisional separability of volume and mass, providing support for models suggesting that both the perception of weight and the decision criterion for perceived heaviness are functions of both physical volume and mass. When visual input was present, results indicated only a lack of decisional separability of volume and mass, providing support for models suggesting that the placement of one’s decision criterion for perceived heaviness is dependent upon the level of physical volume. These findings imply that different models are needed to explain the size-weight illusion, depending upon the type of sensory input available.