[fakedang]

Being Big isn't the criteria - monopolies in that sense are allowed. Exploiting a monopoly position to advantage oneself is what brings the specter of antitrust.

[BlueTemplar]

Being big is what makes economically interesting to do lobbying, regulatory capture, and tax optimization.

[fakedang]

Again, those come under using your size advantage to become big, which is currently not treated harmfully.