Its getting rid of the 2 smaller turbos to make way for a big one
A pair of turbos give good response and low end power, but due to space, you cant get 2 really big ones in there, so your restricted to your top end power - Around 650bhp on the GTRs with a twin setup.
A single turbo means changing the manifold to get either a bottom mount turbo, or a top mount(sits above the manifold) where theres even more space for an even bigger turbo
Singles give great top end power, but are laggy and take a while to boost up
Hope that covers it