One thing ti cinsider is what would have had to happen historically to make them so big. I'd like to focus on Saint John for this. Imagine that sometime in the 19th Century, two things happen to greatly expand growth along the St John River valley.
- Rather than joining Canadian Confederation in 1867, New Brunswick and PEI (and maybe Nova Scotia) form their own dominion. Newfoundland may eventually join. While Canada spends the next century acquiring more and more territory to the north and west (the promise of control over Ruperts Land was a huge advantage to Confederation for Upper and Lower Canada, an advantage the Maritimes get none of), Acadia has nowhere left to expand and begins developing its existing territory.
- With a federal government that cna act freely in its own interest and some already-thriving trade, Acadia experiences the indutrial revolution before central Canada. Factories, urbanization, massive population growth, wage labour etc. all take hold early, giving the dominion a huge head start and ensure that Acadia feels more "european", which will be important for city-building.
The most important infrastructure project, to be undertaken in stages over decades, is what I'm calling the Champlain Seaway. This series of canals, locks and levees will allow ocean-going vessels to travel up the St John and Madawaska Rivers, cutting days off the voyage (in early-20th-Century vessels) from Boston to Montreal. This turns Saint John into an industrial powerhouse, generates tons of revenue and opens both the resources and industries of the rest of the province to new markets. The Seaway's southern terminus is a series of locks connecting the South Bay to the Bay of Fundy, bypassing the hazardous Reversing Falls and keeping the inner harbour for beautiful residential and parkland. Saint John's best analogue would be real-world Philadelphia. The new canal would cause most of the city's industry to relocate to Lancaster, and Carleton to become the CBD. Postwar residential sprawl covers much of the Kingston Peninsula. Historic, working-class neighbourhoods survive in excellent condition to the present day in what is now uptown, the south end and the north end, having traditionally been anchored by the industrial jobs and strong unions along the Courtenay Bay and Red Head. The city's wealthiest residents traditionally live along the Kenebecasis in upper-class neighbourhoods like Millidgeville, until the automobile age when many choose to build spectacular mansions along the Bay of Fundy between Mispec and Saint Martins.
The city becomes famous for its bridges. The inner harbour, no longer needing to be navigable, has five low-level bridges between the reversing falls and the Bay, making sure that both sides of the river are well-connected so that car-dependency is non-existent due to the easy ability to cross the river to go to work, and this culture extends into the far-flung suburbs as well. Even in the car-mad 1960s, 70s and 80s, the majority of Saint John's citizens take public transit to work, echoing the european origins of the city.
Having gone as far as to write all that, I'll just say that Fredericton would be basically Ottawa.