There are however several other large population nations. There's no reason for eg one billion to be a cut-off point or similar. There are only 14 nations with 100m or more people, a sufficiently rare group to look at the nations that fall within it.

The parent's point: "I'm not convinced that landmass and population are fundamental requirements for economic prosperity" - is spot on. They're not requirements at all (just ask Denmark, Belgium, Singapore, Israel, Austria and Ireland) and outsized population is clearly not a booster.

Among nations with at least one million people, the median is around 11.x million population wise. So 100m stands as 9x larger, a very considerable gap in size.

The majority of countries with large populations have done poorly economically. Great economic success with a very large population is a rare exception:

(shown with per capita output rank)

China #67 ($9608), 1.4b people | India #142 ($2036), 1.37b people | Indonesia #115 ($3871), 269m people | Brazil #73 ($8968), 212m people | Pakistan #148 ($1555), 204m people | Nigeria #141 ($2049), 201m people | Bangladesh #144 ($1745), 168m people | Russia #60 ($11327), 144m people | Mexico #66 ($9807), 132m people | Ethiopia #168 ($853), 110m people | Philippines #127 ($3104), 108m people | Egypt #130 ($2573), 101m people | Vietnam #131 ($2551), 97m people

Among the 15 nations with 97m+ people, there are only two standouts:

US #8 ($62606), 330m people | Japan #24 ($39306), 126m people

The poor performance doesn't stop at 100m either, it continues for most other large population nations below 100m as well. Including: Thailand, DR Congo, Turkey, Iran, Tanzania, South Africa, Myanmar, Kenya, Colombia, Uganda, Ukraine, Iraq, Afghanistan, Morocco, Peru, Uzbekistan, Venezuela, Malaysia, Angola, Mozambique, Ghana, Nepal, Madagascar, Yemen, North Korea, Côte d'Ivoire, Cameroon

Between the 100m and 25m population line, you have around 40 nations. At least 26 of those are between very poor and moderately poor.

Here is how the highest per capita output countries rank in total territorial area:

Luxembourg (#167), Switzerland (#132), Norway (#67), Ireland (#118), Iceland (#106), Qatar (#158), Singapore (#175), US (#4), Denmark (#130), Australia (#6), Sweden (#55), Netherlands (#131), Austria (#113), Finland (#64), Germany (#62), Belgium (#136), Canada (#2), France (#42), UK (#78), Israel (#149), New Zealand (#75), UAE (#114), Japan (#61), Italy (#71), South Korea (#107)

Having a lot of territory to look after, is more often an immense burden.