Those are pretty dishonest stats though considering the vast majority of that land is extremely rural and much of it is owned by the Federal government (the Feds own 28% of all land in the country). The UK doesn't have something like Alaska which is both huge and empty (Alaska alone is 7 times the size of the UK, but has just 730k people).
If the US ever had a land crunch, it could easily open up vast new tracks of land for settlement via the US Govt. It's very valuable to have that ability. The Feds are already beginning to sell off their hoard of property due to fiscal problems.
