Reducing the foliage will put less of a strain on the roots after repotting.
I've understood that actually put it under more stress:
"The removal of top growth sends a signal to the plant that there is something wrong above ground. The plant goes into crisis mode and reacts by initiating the growth of dormant buds to replace the leaves it has just lost. This growth process requires energy; also known as a food source. The plant mobilizes stored food from the roots to the new growing points and puts a lot of it’s nutrient reserves into making new leaves.
This all sounds good, except that the plant also needs to grow new roots. This also requires stored food and nutrient reserves. The plant now has to share it’s reserves between the two processes. The net result is that both suffer."
Source:
www.gardenmyths.com/transplanting-should-you-reduce-top-growth/
Seems to make sense to me?
However:
"Having said that, there are some extreme situations. If a woody plant has lost almost all of it’s roots, I might cut it back because without this it will die. The cuts should be made to take out large branches. You want to eliminate a lot of the dormant buds so they can’t grow and take energy from root growth."
So, if you "reduce the rootball a lot", you would not just reduce the foliage mass?