Corporate presidential candidate donations

Why is it that candidates such as Clinton and Giuliani who are backed bigtime by the big corporations only have millions in their campaigns? It seems like between all the corporations they would have Billions.

Remember when General Electric gave Jack Welch a 300 million retirement bonus? That's just one corporation! It seems like all together they could put forth much more money than what they are doing. And why not! It is their profits at stake. Seems like every four years they could together come up with a much bigger "bonus" They pay their CEO's nearly 100 million a year average...

Very strange to me