Most full time MBA graduates from the top schools are recruited straight after graduation so they're not getting a pay increase from the same company. The money is pre-MBA pay in company #1 vs. post-MBA pay in company #2 so you see a big difference. I work for a European MNC and even though they'll pay for an MBA they won't give you a pay increase, however more positions are open to you because of the additional qualifications but even then you don't get anywhere close to 100% pay increment.

Again it depends on your networking and the b-school you go to. Natural progression is there, but for some b-school grads there is the jump to a different industry, function or geography that's made possible by their networking and their MBA. This may include a jump in salary as well, sometimes quite significant.