At the end of 2009, mortgage REIT American Capital Agency (AGNC -0.32%) had around $5 billion in assets. Just three years later, at the end of 2012, that number has grown to approximately $100 billion. Is this insane growth sustainable, or is the bubble going to pop? In this video, Motley Fool financial analysts Matt Koppenheffer and David Hanson compare American Capital to some of the other big mortgage REITs and their growth over the same period, such as Annaly Capital (NLY 0.91%), and take a look at American Capital's track record, to tell us whether the company inspires confidence or trepidation.