The expansion tank does not have the capacity that a well tank would have...
As far as potability issues...
The components of a heating system do not have to protect the quality of the water as it is a closed loop behind a backflow prevention device and will hopefully never end up in your potable water supply.
Take trip over to the drain on your heating system and drain some water into a glass. Take a look at it and see how tasty it looks...
Be thankful that the expansion tank on your potable water system is constructed of materials safe for use with potable water.
A visit to
http://www.watts.com
or,
http://www.amtrol.com/
reading may answer your questions on materials used.