Fractures of the distal radius and the ulna are the third most common fractures in dogs. Toy and miniature breeds have a propensity to develop antebrachial fractures after falling or jumping. Most affect the distal third of both bones involving between 15% and 37% of the radial length. Larger dogs, instead, typically sustain hyperextension injuries to the carpus. The causative mechanisms for this fracture prevalence in toy dogs are unknown. Breed-related changes in bone geometry and/or mineral density have been suggested as possible etiologic factors. In a multifactorial study, the main etiological factors potentially responsible for determining susceptibility to fractures in toy breeds are considered. The aim of this first study is to evaluate the geometric bone features in different dog sizes. The cortical bone cross-sectional properties along the length of the right radius and the ulna of 28 dogs from three different size categories have been quantified by computerized tomographic scanning. Geometrical cross-sectional parameters were measured and normalized to radial length to allow intergroup comparisons. Discriminant analysis was used to classify the observations into different groups. Through statistical analysis of the normalized values, significant differences in cross-sectional properties of different bone sizes were found. The results suggest that, when proportionally loaded, the antibrachii of toy breed dogs are more susceptible to fracture than those of large breed dogs due to morphological differences.