That small dog mounting your leg is more likely than a larger one to behave badly, according to new research from the University of Sydney. The researchers found that “across the breeds, behavior becomes more problematic as height decreases.”

According to the university, the researchers used owners' reports on the behavior of over 8,000 dogs from across 80 breeds and related them to the shape of 960 dogs of those breeds, revealing strong relationships between height, body weight, cephalic index (the ratio of skull width to skull length), and behavior.

The study showed that most undesirable behaviors were associated with height, body weight, and skull shape. 

“Breed average height showed strongly significant inverse relationships with mounting persons or objects, touch sensitivity, urination when left alone, dog-directed fear, separation-related problems, non-social fear, defecation when left alone, owner-directed aggression, begging for food, urine marking, and attachment/attention-seeking,” the study found.

Researchers used results from an international pet-owner survey, the Canine Behavioral Assessment and Research Questionnaire (C-BARQ). The questionnaire consists of 100 items that ask respondents to indicate, using a series of 5-point ordinal rating scales, their dogs' typical responses to a variety of everyday situations during the recent past.

Only three behavioral traits (coprophagia, chewing, and pulling on leash) failed to correlate with a morphological feature, the study found.

"The only behavioral trait associated with increasing height was 'trainability.' When average body weight decreased, excitability and hyperactivity increased," said Professor Paul McGreevy, lead author of the study.

The study, "Dog Behavior Co-Varies with Height, Bodyweight and Skull Shape," was published in the journal PLOS One.