Rarely are Mega Corporations portrayed with anything other than unremitting negativism; rather than being a simple business making things that people want to buy, they are almost invariably the villains of the setting, and depicted as exploitative, oppressive and screwing the rules with their money while maintaining a Peace & Love Incorporated façade. They are home to the Corrupt Corporate Executive, Mean Boss, Pointy-Haired Boss, and Obstructive Bureaucrat, and usually have Amoral Attorneys on the payroll.
Mega Corporations are shown to be private institutions and therefore don't have to play by most rules the government has to, such as freedom of speech, because it's always "nobody is forcing you to work for them or buy from them or use their institutions or buy their products." However, more darker versions will also show these guys pretty much buying off or eliminating their competitors, brainwashing the masses, and coming up with Evil Plans to ensure they have a monopoly and making it so that you still have to buy their products, while their employees are sometimes portrayed as oppressed, paid pitifully low wages (if at all), and treated as expendable.
They may also be shown controlling the government either through having employees in important positions or through lobbying, or taken to its extreme, may have Private Military Contractors or other Hired Guns (or even an entire country or world) at their disposal, and become Superpowers in their own right. Corporate Warfare may result if financial means are not enough to accomplish the company's goals.