Greg posed the question the other day, and I'll paraphrase, that asked why libertarians appear to worry so much about the problems of big government, and not so much about the problems of big business. Fair question. The answer is that we do, but to a great extent we believe the solution to big business is to shrink the government. What we're talking about here, really, is crony capitalism. This is a similar concept to mercantilism and fascism (as an economic system, not a political one).
Quickly, the reason one worries about big government is twofold: public choice issues, and knowledge problems. The public choice problems are that agents of the government can use their office to pursue their own agendas, rather than the agenda that is in the public interest. The knowledge problem is that the government agents do not (cannot) know all the relevant facts of a particular problem, so that their solutions in fact make things worse rather than better. There's boatloads of research on this, I just wanted to get into place for the next step.
The reasons we worry about big business are: monopoly/monopsony power; externalities that cost the public something but the company doesn't have to answer for (think poisoning water ways through pollution); and the ability to 'capture' regulators. I believe these are legitimate concerns, except for concerns over "natural" monopolies since these are unlikely to exist.
Where I'm from, certain utilities used to be monopolists. Some still are. The reason they were monopolists was because the government would only allow one company (in fact the state-owned company) to provide the utility in question. The reason was because it was a critical utility (telephony) and the government didn't want the people to be at the mercy of greedy capitalist pigs (my words). Eventually, the monopoly was ceased and competition opened up. The price of telephone service subsequently dropped precipitously. This trend is ubiquitous when a monopoly is ended. Therefore, this was not a natural monopoly, but one created by the government. Thus we should oppose governments giving monopolies to any business (yes, this included the Federal Reserve being the currency monopolist).
The other concern is that "business" can act as a monopsonist in the labor market and dictate terms to those who want to work for them. I think there are times that has been true - West Virgina coal mines come to mind. But to me it seems the solution is to have more business, not government control. Why? Because with government control comes the opportunity for political entrepreneurship - the big business has the resources to influence politics. I think we can all agree this is not desirable from a people point of view. It benefits a few at the expense of the many. The financial sector is a very good example of this behavior, by the way. The connections between Wall Street and the federal government are deep.
Finally, pollution. Here's another area where big business captured government power. But, I think this pollution issue is going to make for a different direction, so I will carry on later - need to get my thoughts together first.
I hope I have shown that, in fact, libertarians want to reduce government not just because government is bad but because big business feeds off big government.