Research into weight illusions has provided valuable insight into the functioning of the human perceptual system. Associations between the weight of an object and its other features, such as its size, material, density, conceptual information, or identity, influence our expectations and perceptions of weight. Earlier accounts of weight illusions underscored the importance of previous interactions with objects in the formation of these associations. In this review, we propose a theory that the influence of size on weight perception could be driven by innate and phylogenetically older mechanisms, and that it is therefore more deep-seated than the effects of other features that influence our perception of an object's weight. To do so, we first consider the different associations that exist between the weight of an object and its other features and discuss how different object features influence weight perception in different weight illusions. After this, we consider the cognitive, neurological, and developmental evidence, highlighting the uniqueness of size-weight associations and how they might be reinforced rather than driven by experience alone. In the process, we propose a novel neuroanatomical account of how size might influence weight perception differently than other object features do.