i had a similar problem with my audible books app.
there was no way to set a file path for the downloaded books to the 'external sd card' from the app.
and 4 x 200mb audio book sections soon filled the internal memory up!
moving the app to 'sd card' doesn't move it to removable storage. as the 'sd card' is internal storage (handy eh...not)
i moved the files by accessing the 'my files' app.
finding the files i wanted to move inside the audible books folder.
select the menu key/more/move, then navigating to the 'external_sd'
and it gives you the option to create an new folder there and then move the files.
i named the folder the same as the apps folder and it all loads from the external sd card now. even new downloads are dropped there too.