I would suggest it was a combination of efficiency though complex design and lighter weight. The LNER Pacifics weighed only 148-154 tons, and the express trains they hauled around 500 tons at full length, even before the lightweight train craze of the 1930s.

I just blundered into the following while triple-checking the above numbers: By fitting longer travel valve gear to two of the locos (4472 and 4476 Royal Lancer) in April 1928, the coal consumption was reduced from 50 lb. per mile to only 38 lb./mile, making the non-stop London-Edinburgh runs feasible. Do the math: 38 x 400 miles = about seven (long) tons. The rebuilt corridor tenders carried nine tons of coal.

I should note that one book I just glanced at went on (and on and on!) for seven fine-text pages on just the increases in efficiency being engineered out of the locos in that specific era, going as far as calculations of pound of fuel consumed per ton-mile (0.116!). This tends to perpetuate the stereotypes of European-versus-American sports cars design: Europeans extract speed via engineering and efficiency; Americans extract it by brute force.