I think this is a good question, following on from Dave's 'What do you have to do to win that big contract' post.
The biggest job we've had that we 'won' was a Hilton Hotel; and it was only six months after we started cleaning windows. We had to jump through some hoops to get it, and we did, but the job was too big for us at the time (pre-WFP), so we dropped it. Money was tight and they were slow to pay.
But in general, how do you get asked to do big contracts?
A big advert in Yellow Pages maybe?