The problem with the "farther north the bigger" theory is that Helsinki, Finland is the same latitude as Anchorage, Alaska, and most of the 100,000+ moose shot in Finland each year (most with 6.5x55 Mausers by the way) are shot considerably north of Helsinki. Nonetheless, the European moose (Alces alces) of Finland are considerably smaller than the Alaska/Yukon moose (Alces alces gigas) of equal or lower latitudes.
Y'all are probably thinking of the biological "rule" called "Bergman's rule" that states:
In a warm-blooded, polytypic, wide-ranging animal species, the body size of the members of each geographic group varies with the average environmental temperature. According to this principle, warm-blooded animals living in cold climates tend to be larger than animals of the same species living in warm climates.
As with all 'rules', it's more of a "guideline" than a "rule"...
Paul