I think that I would like to showcase some real world examples of how compression is done. For example. take a standard audio CD and “rip” the files into an MP3 of OGG format. Most applications for this purpose allow you to set the sample rate or compression ratio. We can experiment either in small teams or with the classroom digital projector. Students can guess the percentage of file compression and check the results.
Similar work can be done with a PDF file creation, I would also suggest trying this task with image files. Programs that convert/save to JPEG (such as Paint.Net) have sliders to allow the user to select the level of compression. We can determine as a class or in small groups which level of compression is acceptable for a given scenario. Would it work on a phone screen but not be acceptable printed as an 8*10 photo?