In the U.S., the main issue is that the House has had 435 members since 1929. It's become so obscene that representation was better, on paper, for colonial Americans in the British Parliament than today.
I would argue that 435 Representatives is already too much. Humans can only keep track of around 100 people. With so many representatives there is barely enough time for each of them to speak and engage with each other. Increasing the number for "better representation" will just worsen the problem.
A more cynical take is that since they just vote along party lines anyway, the number does not matter too much.
Remember when we used to do things because they were hard instead of falling back on lazy cynicism and convenience of status quo?
The U.S. seems stuck with the two party system primarily due to the mathematics of the Electoral College and lack of ranked choice voting. These require changes at the state level, not federal.
16 states and DC have now opted out of the Electoral College (https://en.wikipedia.org/wiki/National_Popular_Vote_Intersta...). RCV is now in place for state-wide elections in Maine and Alaska, but recently banned (seemingly as a partisan protectionist measure) in Florida, Tennessee, South Dakota, Montana, and Idaho.
Gerrymandering and lack of a mixed electoral system also seem major factors for the political duopoly, with less clear solutions to me.
Also, the more constituents, the more difficult it is for third-party candidates to run and meaningfully affect the debate.
In the end, continent-sized superpower government is really hard. Even in the "good old days", you often ended up with people like Nixon.
In 1900 Congress had 357 representatives for a population of 76 million.
Today Congress has 435 representatives for a population of 331 million.
