Actually I tried this and managed to drop 532mb to 517mb by putting a rar into a rar. I don't know if it'll happen again though
In 99.9% of cases, it won't help at all. The other .009% of the time the difference will be negligable. The last .001% is if it actually helps a noticeable amount.
It's pure chance that yours worked like that, I guarantee that if you try to put that one inside another .rar it will have no helpful effect.