City size is an inherently flawed metric, especially in the US. Hartford itself covers <18 square miles, which is miniscule for a city. That's less than half as much land as the San Franciscos, Pittsburghs, Buffalos, Bostons, etc. of the country that themselves cover 1/5 as much land as a Chicago or Dallas (who themselves cover a fraction as much as the Jacksonvilles and Houstons that just annexed everything around them).
Either way, Hartford's location smack dab between NYC and Boston matters more than Hartford's population. Hartford just can't compete with New York and Boston simultaneously, which puts them inherently behind smaller population centers who would own the market. I'd love few things more than a Whalers revival like the Jets got, but there's a list of reasons why Winnipeg got a second chance while Hartford gets dusted off to be used as a pawn by real estate developers and/or owners looking to dial up the heat on getting a new arena built every few years.
For baseball I think that issue is amplified far more than it is in hockey. The fans who rooted for the Whale and would tearfully root for them again probably have multiple generations of Yankees/Red Sox/Mets/whatever fandom in their households that'd be much harder to shake. Hell, I'm a Pirates fan who does nothing but bitch about the Pirates while living 5 minutes from Dodger Stadium...and I'd rather simply not watch baseball than feel nauseous about being disloyal to an organization that has never done anything to earn that kind of loyalty. Now remember that the Yankees and Red Sox actually try to win things and occasionally to frequently do and just imagine how difficult of a time Hartford would have trying to compete with them.
Whatever issues Montreal has as a market, and it has plenty, are greatly reduced by its size and lack of direct competition. Still have to grow a fan base and nothing grows a fan base better than sustained success, but those are both massive positives.