Size mainly, they are huge countries with lots of cheap land and a historically (and deliberately) high immigration rate.

If you compare population density with those countries and European countries, you will notice that the US and Brazil density is still much lower than almost all European countries (especially their former colonisers), and the US, which is roughly the same size as Europe and Brazil which is 4/5 of Europe, still has less than half the population and a third the population of Europe respectively.

In theory, if the colonizers hadn't arrived, the indigenous population could have reached the same numbers today. So, it has nothing to do with colonizers being outnumbered by the colonized. Just bigger country side in warm lands = more people.