The answers are actually quite short. Corporate surpluses are now at the level of something like $300 billion. They've got to invest that money, so they send it overseas. It can go tax-free in a growing number of tax haven countries. Canada has not yet closed the door on that.
My own personal calculations show that Canada lost $3.1 billion in tax revenue to overseas investments owned by Canadian corporations last year, and will every year. At the same time, I've estimated that another $3 billion is being lost in the domestic tax cuts. So there is a huge amount of money being released through the corporate sector that is draining the tax system quite dramatically.
How does this compare to individuals? Well, when the corporations are not paying taxes, who's left?