There isn’t a lot of detail in your question, I’m trying to get a better understanding of what your trying to do as there might be a better tool than ffmpeg to do this. I’m thinking that ImageMagick and it’s convert tool could be the best tool for the job.
Is there a specific reason for ffmpeg to be the program to manipulate the files?
Is the compression factor the only thing you want to change?
I have a camera that takes 40003000 pixel images, I’d your looking to reduce the file size, would it be acceptable to also use less pixels? Converting to say 20001500 which would still look great on a full HD screen of 1920*1080 pixels.