UAs 2020
City Population updated their urban area list with their very good maps:
https://www.citypopulation.de/en/usa/ua/
Here we can easily see those cases where UAs could be merged. On Boston UA, for example, you have Ipswich UA, which is an enclave and also Nashua, Worcester and Providence UAs with very rigid, straight-lined borders. Arguably, Providence should be a different urban area, but the others might be merged.
Boston UA would have 4,382,009 people in 4,289 km² (density is much higher now with their new definitions). Boston plus Worcester, Nashua and Ipswich would have 5,116,458 inh. in 5,486 km².
And we find similar cases all over the US, aside of course, the classical cases of NY, LA and SF.