Compressing Written Information

To be honest study sheet should be for just things to jog your memory, not to write down real information. You need to study that.

This. Basically, by the time you're tested, you should have already learned most, if not all of the information. There should not be any significant portion of the material that is new to you. The only things you should be writing on your cheat sheet are details that you can't figure out through reasoning, things that you are actually confused about (not even "might be confused about"), and possibly any formulae that you've devised that can save you time. Therefore, your cheat sheet should really be treated only as "data sheets", because after all, the cheat sheet is meant to save your time, not consume it.

Only if you have extra space should you fill your cheat sheet with random junk that you probably don't need, but you might find helpful.

Here's an example of the cheat sheet I used for one of my Gen Chem 2 exams:

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If you write small and only what you need to write, you should not run out of space, and you should not need to "compress" the information to such a point that you need a legend to figure out what you're trying to say.

As an experiment, try producing such a cheat sheet, where you're trying to write literally everything down. Then read it from front to end and see how long it takes. If it takes more than 10 minutes to read the entire cheat sheet front-to-back, your cheat sheet is more hindering than helpful, especially if your exam is only an hour long.