Cover images are handled completely differently from the way images are handled in the body of either reflowable or FXL EPUB. There is basically no way to get a full-screen image like the cover.
You can use individual object formatting on each image to get more options than the general export offers, though. Click on any image to select it, right-click to bring up the context menu, and select Object Export Options. What you probably want to try first is setting export size to "None," which makes most readers scale it to the maximum size their interpretation code allows.
Here's more detailed information you might find useful:
https://nitrosyncretic.com/DPR/dpr_indesign_epub_image_mgmt.php
If you're fairly new to EPUB, or EPUB-from-ID, you might find this general primer a good starting point (before the above):
https://nitrosyncretic.com/DPR/dpr_indesign_epub_basics.php
Ask away if this leaves you with any questions.