Image Conversion problem from Epub to AZW3

[rosshalde]

Hello,
Using Calibre to convert from an epub created in Sigil to AZW3 for Kindle Fire.
Images are too small. I can double click them to fill the screen, but am I doing something wrong in conversion. I even tried taking a larger version of the same image and replacing the smaller image, but I still have a small picture.
Perfect example is my cover picture. It fills the page when converted to mobi and viewed on standard kindle e-reader, but is tiny on Fire HD6. Any ideas?
I am sure it is something I am doing wrong, as I am very new to all of this.