It is safe to uninstall. Nothing is required from old versions for new versions to run. The only thing is to run your projects in 4.17 and ensure you’re happy it all runs without issue. Nothing will be horribly broken (Usually), but sometimes nodes get updated so you have to do a few fixes.
Just a note, part of the size difference might be the optional component. If your using the Launcher. Press the dropdown button next to launch, and press options. You can choose optional extras that perhaps you had on with 4.15 that are not on by default in 4.17. You can tick stuff on you need and untick what you don’t to save disk space.