The FICO score does not factor in mortgage loan balances. If it were to do so, then it would have a disproportionately negative effect on those in who buy in higher priced areas.
Potential creditors, however, may look at your mortgage loan balance to determine whether they consider you to be an appropriate credit risk. Having high mortgage balances can make it difficult to obtain new credit, even if your scores are high.
I have personal experience with both situations. I have a jumbo mortgage, and shortly after it was opened my scores were actually higher. So my scores weren't affected by the large loan balance. However, despite having high scores, I just had a CC denied because of a high loan balance.
