An interesting thought experiment. Not that it would ever happen, but it is fun to imagine how things might be if we were twice, 10, 20 times our size. I mainly know NB so that's what I'll muse about, lightly touching on NS and PEI along the way.
For the Maritimes, the Fundy will basically fill in and become a megalopolis likely. Freddy would be an outlier (going up to Woodstock and growing up the Nashwaak river along route 8 basically), but the centre of the province will still be fairly empty. A corridor would fill in between Freddy and Saint John, not quite following the river due to CFB Gagetown (but there would likely be pressure to move the base somewhere else from Oromocto and SJ boundaries squeezing on it). Freddy would mainly grow south (to Freddy Junction) and west (along the valley, absorbing Nackawic). South east it would follow the river somewhat but the flood plain would keep it less developed. Northward it would grow north west through Keswick and Mactaquac, and north in general, with major development spurs along Route 8 and Route 10.
Sussex would be a major city in its own right, and probably be NB's "hub" city being central to the bigger metropoli of Freddy/SJ and Moncton. Route 1 would likely be a major 3-4 lane route between Moncton and SJ.
Route 2 would probably be a 3 lane highway, but would finally be rerouted to go to Sussex instead of the Irving hinterland. Gagetown, Coles Island, Cambridge Narrows would be decent sized towns in their own rights. Salisbury, Shediac and Sackville would be all absorbed in the greater Moncton municipality/city.
Amherst would be a smaller city, feeding off of the proximity to Moncton while being in a different province. Truro would be a major Nova Scotian hub city. Wolfville and Digby will likely be decent sized cities, serving the University and the Fundy coast. There would be regular (multiple trips daily) highspeed ferry trips between SJ and Digby.
Probably 6 lane highways between SJ to Moncton to Halifax. 4 lane highways all the way to Sydney, including expansion of the canso causeway. Antigonish, New Glasgow, Port Hawksbury would all be decent sized cities (probably on par with current Maritime city sizes) if not bigger. Sydney would finally be pulling its weight. There'd likely be twice the number of ferries (high speed ferries maybe?) across to Newfoundland as well.
The Confederation Bridge would be expanded, with rail lines and at least 3 lanes each way. There might even be talk of a second crossing around Pictou. I'm afraid I can't even guess how the rest of the island might change.
There would certainly be regular passenger rail service between SJ and Halifax, probably a regional GO-style rail system between NB's tri-cities, with passenger rail service to Charlottetown and Sydney and the northern cities. Probably not high speed rail service yet between SJ to Halifax, but it would be heavily studied and perpetually "Coming soon".
Moncton and Halifax would be fairly major international hubs to serve the millions in the Maritimes; Halifax still bigger, but Moncton drawing more and more through the years, as Halifax has growing pains, and Moncton's maritime Hub location being more appealing. Freddy, SJ, Sydney and Charlottetown airports will be major national airports, with a few choice international destinations, but mainly handling flights around the region and to Canadian hubs and a few US destinations and sun destinations.
The SJ valley and central NB would likely still be a lot of towns. Bigger towns; Woodstock and Grand Falls would be cities akin to Freddy or Edmundston currently, but still towns. Edmundston would probably be close to current Moncton in size (not really thinking hard on the numbers to be honest) and serving as the entrance to the Maritimes. Bathurst and Miramichi will finally be decent sized cities, still small compared to the others, but the shrinkage would mostly be stopped and they would probably see a decent tourist destination, especially with French tourists.
There would be a four lane highway looping all around the north part of the province, connecting the cities together, along with regular passenger rail service. Route 8 (Freddy to Miramichi to Bathurst) would also be fully twinned. Route 108 (Miramichi to Plaster Rock/Grand Falls) would be upgraded to highway but probably not twinned yet; Route 2 (TCH) and Route 15 (East coast) would be the major North-south routes in the province.
The east coast of NB would be much more heavily urbanized and french, Bouctouche being a small city on the verge of being absorbed into Moncton, and Richibucto being a major stopping point between Miramichi and Moncton.
St Stephen, Woodstock, Perth Andover, Grand Falls, St Leonard and Edmundston would all be major US/Can border crossings. A couple of highways and rail lines would finally be pushed through across Aroostock county, linking St Stephen and/or Woodstock more directly to Quebec.