Hi casaundra208,
Thanks for posting your issue here. We'd be glad to check this out with you. We understand that the photos for your contacts aren't always displayed in full screen when you receive a phone call. There’s a few reasons explaining why this would occur. The size of your images being displayed depends on the resolution of the image that was saved to your contacts. If the image has low resolution, was cropped too small, or zoomed in, it wouldn’t display as full screen. You can try testing this out with different images from your camera roll, or taking new high resolution photos with your iPhone.
In addition, if you’re saving your contacts through a third-party account, the images could also be uploading at a smaller resolution. To check to see where you have your contacts saved, head to your Phone app > Contacts > Groups, and see if you have other mail accounts listed with contacts saved in them. If you do, try recreating the contact in iCloud instead, and upload an image with high resolution to it. To make sure new contacts are being saved to iCloud, go into your Settings app > Contacts > Default Account > choose iCloud.
If you’re still experiencing issues after this, we’d recommend reaching out to an Apple specialist via Get Support - Official Apple Support.
We hope this helps! Take care.