The thing governments try to intervene to stop is anticompetitive behavior. Monopolies easily lead to this, so we single those out and stop them early, but that's just one situation that leads to anticompetitive behavior. There are many aspects of megacorps that also can and do lead to anticompetitive behavior, all that remains is to determine if additional categories of behavior that are more common today also rise to the level that we want them specifically singles out and regulated as well.

Monopolies aren't inherently bad, they're regulated because they incentivize anticompetitive behavior that hurts consumers and the economy in the long run (you have to assume companies will act against their best interests for them to not cause problems). Megacorps aren't inherently bad, but we may want to regulate them because they may incentivize anticompetitive behavior that hurts consumers and the economy in the long run.

The problem is that “too big” becomes synonymous with monopolistic behavior. It’s the nature of our system that drives them to that end. Winner takes all.