I have two dwellers at Level 50 with all their stats fully developed and their happiness at 100%, who I send out to the wasteland.

However they always return after 4 - 5 days max stating that what they have collected is all they can carry.

I have read in relevant topics about dwellers lasting 15 days out there and I was wondering how this is done.

That was before update. Now they collect the maximum number of items and then return home.

My max is 3 days then they die