The amount of data needed to store a rune would indeed be even smaller. No need to store images or even references to images. You would only need to store the number of stars, rarity, empowerment level, and the amounts for each primary and tertiary to extrapolate how the rune will grow during fusion / empowerment (if not maxed already). Since the max numeric value for any given bonus is relatively small, you'd probably only need 4 bytes for each value. You could further avoid using character field by assigning integer values for every possible word or phrase used to describe an attribute or bonus (0 - green, 1 - blue, 2 - purple, 3 - orange and 0 - starting energy, 1 - movement speed, ... etc). I'm sure Perblue put thought into this when they designed the database tables for runes.
I'm guessing the real stumbling block is re-working the UI for the Rune Temple, Rune Shrine, and possibly other places to optimize fetching the player's runes and avoid long loading times or lag when viewing runes.