Medium is generally enough unless you carry lightweight clothes, but less dense stuff can generally be compressed. The large ends up being bulky and if you pack normally you will be well over 22-24kg limits. Also the chances that someone (e.g. airport/airline staff that doesn't give a damn about your bag) will damage a 25kg / 60 lb+ bag is much higher. Plus, as long as you get an expandable bag, mediums should be more than enough.