Your responses, while hilarious, were nowhere near the truth - with the exception of commenter Tannim Murphy. There's a very simple way to make the rainbow circles make sense. Here's the key.
If you noticed, the circles are drawn on a grid. Were you to count the number of squares that measure the diameter of each rainbow circle, you'd find the range of each AoE healing spell (I assumed maximum glyphs and talents).
The original intention was to measure the surface area of each heal. Unfortunately, the dynamics and characteristics that make each heal different and unique make it really hard to compare them to one another fairly, let alone measure surface area.
Chain Heal, for instance. Chain heal doesn't look anything at all like a rainbow circle. It looks, ironically enough, like a CHAIN. Imagine that.
Methodologically speaking, you can't directly compare Chain Heal to the other AoE healing spells, because it doesn't function the same way. It's linear instead of radial.
Divine Hymn is channeled. It works on ticks, and therefore it's unlike many of the other spells - You don't have spellcast -> spell effect.
In order to really compare all these spells, you'd also need to think about the differences in cooldowns, mana cost, and the amount of time it takes to cast each spell. The spells are also different in that some affect only one's party instead of the whole raid. Some are smart heals. Some, I have no idea how the UI decides who gets the heal (i.e. Holy Light glyph splash).
And thus, I gave up on the rainbow circle AoE heals post, because after spending hours creating my rainbow circles graph (not to mention days spent searching to find the perfect software program with which to create it), I went and confuzzled myself with all the ways that my methodology would not function properly. And then I abandoned it.