Problem with resizing book covers.

Hi - I have installed Resize Cover as a plugin alongside Polish Books with a size of 750 width 1081 height, but when the books are shown in the Kindle app on a Lenovo Duet 3 tablet, the books sizes don't match with some books looking taller, some wider, and some small. Please tell me what I am doing wrong. I have tried using Epub and AZW3 formats but nothing's working. Please tell me what I am doing wrong.

Thanks,

Gavin.