In fact it peaked in size in 1920. But you’re broadly correct about the sense of decline post-WWI and the sense that America would be the dominant 20th-century power.
The UK itself lost one third of its land area in 1921, following yet another insurrection in Ireland (always a coerced part of the UK subject to genocidal rule and expropriation).
The last straw for the Irish was summary executions of the insurrectionists and the ravages of the Black and Tans -- like ICE but with arsonists and criminals, but without masks. The US will be fortunate if the parallels remain only financial.
