The bigger your balance relative to your credit limit, the lower your score.
But while it may be tempting to close out a credit card account when you transfer the balance to a lower-rate card, you may inadvertently hurt your score. That's because your total balance stays the same but your credit limit goes down when you close an account.
Say you have three credit cards with a combined credit limit of $24,000 ($8,000 each) and you owe $6,000 total. Your balance represents 25% of your credit limit. If you then close out one of your accounts, your credit limit goes down to $16,000 but your debt is still $6,000, which now represents 37.5% of your credit limit.
To boost your score: Don't close unused accounts when you transfer debt.