I mean corporations in the multi-billion to trillion dollar definition rather than a small mom-and-pop "corporation". For one I think antitrust legislation should be more thoroughly prosecuted - most notably I think network effects beyond a certain threshold should not be legal. For example, Hacker News has some kind of network effect - network effects are invariably going to happen in society.
But the network effect that Instagram/Facebook/Discord/Twitter/etc. has reaches so far that any teen or young person that wants to be part of digital life must use these corporate ran companies. Having a stronger world wide web and a weaker app based internet would water down some of these network effects.
Automobile companies have network effects because of their historical lobbying that made teen life in the suburbs nearly impossible without a car. I'd like to remove money from politics to remove a lot of these incentives big corporations have to protect their bottom line.
Now... what about a regular company like Nike or Proctor and Gamble or UPS? I think the way you don't create mega corps like those is to prevent globalization with these companies. Keeping supply chains within their own countries would help, for example. Keeping companies like Meta operating within the US or even within California only would create much more competition, and make many smaller companies much wealthier. An Instagram like app operating in only New York would make it much easier for social media competition within New York because there would be limits to how large that app can get. Trying to compete as a new social media company with Instagram is nearly impossible when you're up against a multi billion dollar company with a billion users.
These are all just random theories from some guy (me) on the internet, who knows if any of these would actually work. But I guess the point is I believe in more smaller markets rather than fewer, larger markets.