As for the crossing from NZ-Antarctica, even large planes struggle, there's generally a point in the journey that they have to commit to landing as they don't have enough fuel to get there, abort and come back. This can be an issue as the weather in Antartica changes rapidly so you may find yourself in a storm after getting the green light to commit, as the RNZAF 757 did a few years ago:
Impressive as they are, the last(final?) time the C-5 came to Christchurch the C-17s looked ordinary next to them.
When the C-5 was lined up, the taxing A320's looked like dinkys.
However, the C-5 was to big to head down to the ice.
I think this has been posted here before but: https://www.cbc.ca/news/canada/calgary/kenn-borek-smithsonia...
As for the crossing from NZ-Antarctica, even large planes struggle, there's generally a point in the journey that they have to commit to landing as they don't have enough fuel to get there, abort and come back. This can be an issue as the weather in Antartica changes rapidly so you may find yourself in a storm after getting the green light to commit, as the RNZAF 757 did a few years ago:
https://www.stuff.co.nz/travel/news/65543703/murray-mccullys...
(I'm not sure if the C-17 planes the USA fly down there have the same issue but every other plane will)