Beatles are still the biggest act. The best selling album last decade was 1 by The Beatles
When a 50 year old band still gets the best selling album of the decade (1 has 35 million and counting sold) they're obviously still eclipsing everyone else.
Population differences? Don't matter. Even with 2 billion or more people existing on this earth since The Beatles, One Direction STILL doesn't have the fame that The Beatles had or even still have today.
There is no way to make 1D look bigger than TB, it's simply illogical and unfactual.