There are few true dwarf fruiting citrus, and none of them are Grapefruit. Instead, they are grafted on a rootstock that makes their structure develop more slowly. If not pruned, they will eventually catch up in size to their ungrafted "standard" counterparts. Here's some more information I've compiled about Citrus:
http://www.plantsthatproduce.com/gui...mmonCitrus.htm