At what point is a business making decisions for its own interest not a problem? We always criticism big companies for their market position. Where is the line

At what point is a business making decisions for its own interest not a problem? We always criticism big companies for their market position. Where is the line drawn between big enough to be successful but not big enough to be unfair player using existing business to leapfrog into other areas? It seems we are quick to point a finger but its never qualified. We just think "x is big, x is doing y, x is being unfair."