With the extra clarification that just the size of the company or its market share aren't in and of themselves enough to constitute a crime. It's how the company acts once it has that power that constitutes the crime.

You could be a huge company, or dominate a market and still not run afoul of anti-competitive laws because you didn't abuse the position [0]. The abuse of position particularly (or only, depending on jurisdiction probably) if it brings harm to consumers is what lands a company in hot water.

[0] https://thehustle.co/originals/the-worlds-cutest-monopoly