Airports have what are called minimum connect times (mct's). These are guidelines that the airlines follow so connecting passengers will have time to get from flight to flight. Now these widely vary depending upon airport, terminal, connecting to another airline, domestic v int'l, etc. Typically smaller airports have smaller mct's due to the fact that you don't have to walk as far, baggage can be transfered easier, and so on.
Airlines build in to their reservation systems these mct's, so if a passenger calls reservation they won't be booked on two connecting flights they don't have a chance of making. If you have a 20 min connect I would hope that it is either a small airport, close connecting flight, or it is a through flight. If you are worried about it I would call the airline and make sure this is supposed to be done and not a break-down in their res. system.