I wasn't suggesting that we scrap the Antitrust Division of the US Department of Justice specifically. You make a good argument that we still need them. However, there are many other areas in which the government could be scaled back without incurring a robber baron scenario. Do some googling for "regulatory capture" and you'll see what I mean. Simplifying the tax code and the bureaucracy that comes with it is one good candidate.
I've noticed that most examples of regulatory capture provided in the Wikipedia article involve regulatory agencies failing to enforce rules on selected companies (e.g. BP was allowed to drill in the Gulf of Mexico without doing the environmental legwork, Goldman Sachs was allowed to manipulate the commodities markets, etc.), which resulted in Very Bad Things happening.
These cases don't show that "big government" regulation is bad. They show that failure to regulate is bad.
Off the top of my head, I can't think of too many examples of companies using the government to their own ends. OTOH, I can think of many examples of government restricting the freedom of corporations in the name of some social good. For this reason, I think that if we scaled back the government, corporations would have more power, not less. (Not to mention other negative effects of government abdicating its responsibilities in particular areas.)