I have tried to drop fields as you suggested. and I am not dropping any key fields and the result is the actually the file size goes up. I was using binary load and then drop a few unused fields. when some fields were dropped, the file size does go down a little bit (very minimal). but for some fields when they are dropped, the file size actually goes up by as much as 20%
Well do not worry about this too much. What version of Qlik are you on? I remember there was a bug in one of the versions which was causing behaviour you have described (that after dropping data the actual size of file grew or was not changed)
Just keep dropping EVERYTHING what is not used. From my experience i could usually reduce size of the app by 60% and significantly improve performance.
Would you be able to scramble data in your app and attach qvw here so i could have a look at it?