misunderstanding
The number of impressions an ad gets is governed by 3 things:
1. The type of targeting and match type.
2. The inventory of search terms to display against given (1).
3. The daily budget of the campaign the ad is in. If Google 'gave' my small business ad as many impressions as an advertiser like Ebay gets, my bill for a single day would bankrupt me and my website would probably go down from all the traffic.
The budget part is the one you are missing. Google forecast the cost per click for a particular campaign and the average clickthrough rate. Once they have these numbers it's pretty easy for them to calculate the amount of impressions they need to serve so the customer spends their budget. If they went vastly over this number the customer would overspend.
The AdWords system does contain some opacity but not really around impressions. If you up your budget and you are advertising on keywords with a large inventory you can have more impressions. Obviously a larger company with more spend can buy more billboards, radio spots or newspaper ads, it's the same online. The difference with online advertising is that it is possible for the small advertiser to pay the same price as the large one for EACH spot.
And for those who all say they don't know anyone who clicks on an ad I can tell you that approximately 25% of searches on commercially interesting query terms result in an ad click.
Disclosure: I used to work for Google. I now run my own company which does search marketing for clients with Google, Microsoft and Yahoo! among other things.
G