When does big become too big?

First of all, various national governments have grappled with this question since the beginning of the Industrial Revolution. They've evolved various tests to try to determine the appropriate point to intervene, but I don't believe these tests are either consistent from nation to nation nor are they adequate even within a single nation.

USDOJ pursued AT&T for years before obtaining the breakup agreement in the 1980s. The telecommunications industry is nearly as concentrated and just as non-competitive now as it was before.

USDOJ pursued IBM for years (thought to be one of the reasons they didn't write their own OS for their microcomputer ["PC"] products), until the company basically shed enough businesses that they were no longer interesting.

USDOJ pursued Microsoft, but around the time of the 2000 election, they decided not to break the company up.

I think the (recent) history shows that no one in Washington really knows where the line is or what penalty is appropriate, or even how long a penalty should be enforced.