Well....epub makers often insert page breaks at the beginning of each new chapter. A mobi ebook will not create those page breaks and will create one long, run-on file.
I'm not sure that will explain 144 pages...it depends on how many chapters are in the book, I guess! Plus, there could be simple things like the dedication, author notes, etc, that were each on single pages that are now not.
What are you basing your page count on? Is it Kindle pages? (I don't have a Kindle, so I can only answer in general on what I see when I convert epub > mobi).