Mmm, and a technical question

Someone brought muffins in to work for everyone to share today. Chocolate ones. I am vainly trying to pretend that these are actually nutritious food and not just giant cupcakes. :-)

Also, I have one of those stupid techno questions that will have everyone rolling their eyes at the ignoramus. It's about file compression. For years I've downloaded and opened files that have been zipped or rar-ed, and all this time I have assumed that the files had been zipped or rared in order to make them smaller and easier to upload/download. With this idea in mind, I decided that since I had a large file I wanted to upload, I would rar or zip it first so it wouldn't take so long to upload and be easier to download by its recipient. So I do the rar thing and the final product is almost exactly the same size as the original file. Confused, I then try to zip the original file, with the same result. The 'compressed' file appears to be virtually the same size as the original. Am I doing something wrong or have I been mistakenly assuming all these years that zipping or raring a file will make it smaller? (And if it doesn't, why do people bother?)