Countries which could have much bigger populations

  Small quibble as the US population, at least according to wiki is "over 333 million" so its more like a tripling but that would still be far too much as it would mean ~90% of world resources going to the US alone.  [Remembering also that I suspect this resource consumption probably excludes indirect consumption via imports of goods made elsewhere].

  Furthermore the US was once very heavily forested but the vast majority of this has already been cleared, largely in the regions east of the Mississippi.

  With Brazil also rain forest soils are notoriously poor.  The bulk of the nutrients are held in the plant cover and once the relatively thin soil is exposed to sunlight it will quickly erode away.  You can delay the problem for a bit if you can burn much of the forest cover as the resultant ashes give a brief period of considerable fertility.  That's why slash and burn methods in such territory works for small migrant tribes moving from area to area and leaving the vast bulk of the forest untouched to close up the wounds but it doesn't work for large scale clearance.  Which is doubly stupid when done for something as inefficient as cattle ranching.

  Ukraine could possibly support ~100M people, although it would have a much smaller [peacetime] grain surplus.  It might have reached something like those figures even without migration in from Russia if it had avoided the Russian civil war, the deliberate famine under Stalin and then the horrors of the Nazi conquest and the bitter fighting over the region.

  Possibly but, unless you assume different development or higher tech then the climate, plus the more attractive economic position of the US to the south makes it difficult to get mass settlement in Canada and have them stay there.  Even if you have global warming get very bad you still have the problems of the higher latitudes having shorter and weaker daylight for both crops and simple human and economic efficiency.