Why some files have more compression than others?

Usually, the bigger the size - the higher compression ratio. Each file would have a slightly different compression ratio, depending on how ‘busy’ is the image: some the areas with low details can be compressed well, and areas with high details - not so well. So a file of 17MB is likely to be compressed to 15MB, because most pixels are the ‘signal’ ones that bear information.

This is because some level of compression was already applied on-camera during shooting as designed by camera manufacturer. While a file of 86MB could be compressed to 20MB, as in the original there is more ‘noise’ (pixels bearing no information) which we remove.

Rawsie is really a hybrid of pre-processing + lossless compression, so that if an image is naturally rich in information, that information still needs to be preserved in the file.

Also, please bear in mind that some manufacturers have already implemented lossy compression in-camera, and this lossy compression has modified the raw image beyond what we consider the maximum acceptable modification, so that there is not much we can do.