I am not sure I understand what you mean by being aggressive on a “physiological level.” Although our thoughts naturally precede our actions, aggression is typically expressed through our communications and conduct. Other than in-depth psychological testing or analysis, behavior is the only way to effectively measure aggression.
It is a common assumption that vegans are more peaceful and less violent than meat eaters, but there is no compelling evidence to support this as yet. Certainly this can be disproved observationally if we consider that a number of associated activist groups (vegan, animal rights, and/or environmental) have been verbally abusive toward people who disagree with them and physically destructive of property. There are undoubtedly vegans who are so profoundly committed to their beliefs that they will do anything, including violent acts, to impose their views on others in an attempt to force social change.
I have personally known quite a few angry, aggressive, volatile vegans, as well as many kind, compassionate, and generous meat eaters. It seems that personality has more to do with these types of qualities than diet. Of course, it can’t be discounted that the majority of vegans were initially inspired to change their diets and lifestyles because of a commitment to ideals that reflect peacefulness and empathy. Therefore, if indeed vegans are less aggressive than meat eaters, it would seem that the principles that motivated their veganism were established well before any practical behavioral changes were effected by their diet. This may simply mean that, in general, peaceful individuals are attracted to veganism, rather than a vegan diet being the impetus behind nonaggression.