multi row card Title

Power BI has think kind of display behavior that if there is only one text field, it would make the text size bigger and look like a title of the Card (which actually not). If there is more than one text fields, to keep all of text fields in the same style, it would use normal text size to display all of them. This is secenario you see on your environment.

The workaround is to use Title for the Card. The title size and style would not be changed when you add more fields into the card.