The domestic dog shows a wide range of morphologies, that humans have selected for in the process of creating unique breeds. Recent studies have revealed correlations between changes in morphology and behaviour as reported by owners. For example, as height and weight decrease, many undesirable behaviours (non-social fear, hyperactivity and attention seeking) become more apparent. The current study aimed to explore more of these correlations, but this time used reports from trained observers. Phenotypic measurements were recorded from a range of common dog breeds (n = 45) and included cephalic index (CI: the ratio of skull width to skull length), bodyweight, height and sex. These data were then correlated with results from the Dog Mentality Assessment (DMA), which involves trained observers scoring a dog's reaction to stimuli presented over 10 standardised subtests. Each subtest is designed to evoke a behavioural response. Backward elimination and weighted step-wise regression revealed that shorter dogs demonstrated more aggressive tendencies, reacting defensively toward both assistants dressed as ghosts (p = 0.045), and to a dummy (p = 0.008). Taller dogs were more affectionate when greeting and being handled by humans (p = 0.007, p = <0.001, respectively). Taller dogs were also more cooperative (p = <0.001), and playful (p = 0.001) with humans than shorter dogs. Heavier dogs were more inquisitive toward a dummy (p = 0.011), to the source of a metallic noise (p = 0.010) and to an assistant (p = 0.003). Heavier dogs were also more attentive to the ghosts (p = 0.013). In comparison, lighter dogs were cautious of a dummy (p = <0.001) and fearful of the sound of a gunshot (p = <0.001). Lighter dogs were also cautious of, and demonstrated prolonged fearfulness toward, the source of metallic noise (p = <0.001, p = <0.034, respectively). With a far larger sample and the advantage of third-party reporting (which overcomes potential owner bias), the current findings build on previous studies in this field, further supporting covariance between morphology and behaviour.