I am hoping that was meant to be funny!
On the topic of population,
wikipedia notes that Portland's highest population was for the 1950 census, with a population of 77,634. My understanding is that a lot of the WW2-era growth came from the jobs provided by the ship building industry (primarily in South Portland). Plus you can factor in that families each had more children back then. I do think we could reach that population number again in several decades, but it's hard to imagine it going much higher than that given the factors already sighted such as geography and economics. I appreciate the observations about how including surrounding municipalities such as Westbrook and South Portland puts us over 100,000. That seems fair to me, given that Portland is basically the downtown section of those surrounding cities.
While mega projects like Maritime Landing add a lot of households in a dramatic fashion, a stable growth rate seems more sustainable and practical. An alternative to building "towers" to house people would be continuing building on the scale that has been followed historically in the downtown area like 5-6 story buildings. Part of what impresses me the most about most big cities, like New York, Boston, and Chicago, is the density of the non-skyscraper neighorboods. Portland's Parkside neighborhood is not our most picture perfect neighorhood, but the built environment here allows for a population density of roughly
13,000 people per square mile without any buildings over 5 stories.