If you expand the radii around Detroit and Boston to 250 miles, "Boston" picks up New York, but "Detroit" makes up for it with Chicago, Toronto, Pittsburgh, Cincinnati, Columbus, Indianapolis, Dayton and arguably Milwaukee (depending on where you measure from, it's just about 250 miles). Those 8 metro areas alone would add nearly 25 million.
BTW, Cleveland's radius is actually higher than even Detroit's. Within 150 miles there are 19 Metropolitan Areas and 27 Micropolitan areas with a total population of 17,018,140. The largest are: Detroit - 4.45 million, Pittsburgh - 2.43 million, Columbus - 1.61 million, Akron - .70 million, and Toledo .70 million. If you include Canada (Hamilton, London, Kitchener, Windsor, Sarnia, Brantford, Chatham-Kent, Norfolk, Woodstock, and Stratford) the number jumps another 2,419,276 for a combined total of 19,437,416 which gives Chicago a run for its money.