It is not normal that other poeple told me that with the same number of images and 300 more pages his final file is 1.8MG and my file is 4.48MB
That measurement is not reliable and can very wildy based on the image quality and compression potential. Besides, 1.8 mb to 4.5 mb is not a big deal to worry about.
Is there any way to reduce the .MOBI file size??