The compression in raw files is lossless. The reason while the files are different sizes is that some subject are more amenable to compression than others. Also higher ISO introduces more noise, which increases file size.
Howver, all these images are the same number of pixels 3072 x 2048.
There is also an embedded JPG, which varies according to the settings in the camera and the subject matter. The JPG can be different pixel dimensions.
Regards,