Can my subtitle be bigger?

Yes, however, subtitles are meant to be small. Take a look at book image thumbnails on Amazon.com. The golden rule of book cover design is to ensure the book cover appears to belong in the marketplace. If we increase the size of the subtitle, the cover may start to look a bit amateur, and we don't want that! The subtitle is important; that's why it'll be inserted into the book's metadata, but it should be small in the design.