If it's relative to geographical size and/or population, there is simply no room for doubt. Puerto Rico by a mile, both currently and historically.
how many fighters are from foreign countries but are managed in the US and train in the US with US trainers and fight all of their fights in the US?
Over the course of modern boxing, no one has come close to the United Kingdom. We held all belts in boxing for over 130 years, a record that posssibly will never be broken.