By no means is that relationship absolute---there are lots and lots of factors that help determine voter turnout, ranging from the importance of the issues in play to the strength of local voting tradition to the personalities of the candidates to the weather on election day. But looking at the evidence, it is apparent that size of district is an important variable.
Bobby O'Keefe of the Atlantic Institute of Market Studies looked at information his organization has collected and came to much the same conclusion. You can read his analysis of ten Canadian cities with populations between 200,000 and 500,000 here. For a larger database of Canadian cities, click here.