My guess: Big corporations often have more potential to outsource, and are better known, and have a stronger PR and legal department. These traits give them a much better negotiation position (don't remember that governments profit from taxes, so it's a bit of a demand and supply thing: If taxes are high in one country, a big company will threaten to move to another, which costs the country money in the end).

At least, this is the argument that the Dutch government uses to abandon as many taxes for companies as possible.