TL;DR: dropping non relevant information from a message saves data. Slightly misleading title and article. Nothing about lowercase saves data. Not having two (

TL;DR: dropping non relevant information from a message saves data. Slightly misleading title and article. Nothing about lowercase saves data. Not having two (or more) different letter cases saves data; if you had ALL UPPERCASE DATA the effect would be the same. And, the fewer other chars you use, the best - stay on lowercase or uppercase ascii only, and your compression will be better. Having "A" versus "a" conveys information; if you drop that information, your message can be naturally smaller. What if you actually needed to differentiate jack from Jack? Only using lowercase letter you may be forced to do something like _j_ack, which would probably take _more space_. (information theory discusses those topics, especially the information enthropy concept).