As banks as diverse as JP Morgan Chase & Co. and Discover Financial Corp. offer hints about how the biggest bank accounting change in decades will affect their bottom lines, a common theme is emerging: credit cards.

If a bank has a sizable chunk of credit card customers, new loan loss accounting rules coming into force in 2020 will hit them harder than banks with higher volumes of short-term commercial loans.

The reasons are nuanced, but it comes down to...