The reason the cover image is small is because you used Book Designer to make the PRC instead of following HarryT's tutorial. Book Designer does a so-so job of making Mobipocket eBooks. BD tends to reduce the images in size including the cover. Plus it doesn't put in links for the cover, the ToC, and to start reading. Using the tutorial works so much better. Then you can post version 2.0 and from there use mobi2imp to make versions for the EB1150 and EB1200. See the link below for the tutorial thread.
https://www.mobileread.com/forums/showthread.php?t=11678