Posted 21 January 2013 - 10:36 PM
The image you posted immediately above *appears* to be a drawing, not a photo (but I could be wrong.) That notwithstanding, the Moon's disk is blurred, not sharp-edged, and so could be taken to be due to diffusion of light by the cloud layer. The lunar disk will be quite a bit smaller than this fuzzy blob of light.
A photo which clearly reveals the dimmer halo will necessarily greatly overexpose the Moon, which even in the absence of cloud could result in something of a 'bloating' effect. And the scatter induced by the ice crystals will only accentuate this.
The math is pretty clear. A 1/2 degree Moon can only be 1/88 as large as the 44 degree diameter halo. It would be instructive to make a correctly scaled sketch to see this.
In your original drawing, it's not that the halo is undersized, but rather that the Moon is oversized. This is a virtually universal mistake. Look at how huge the Moon is usually made in the movies, for instance. The root cause is this. The very bright Moon appears relatively larger than it is compared to dimmer phenomena.
Try this. Next night out, look at the two pointer stars in the bowl of the Big Dipper (which point the way to Polaris.) By your best estimate, how many full Moons do you suppose could fit side by side between these two stars? As for myself, even knowing the answer I have to really convince myself that some 10 Moons could fit, not the 4 or 5 that my eyes mistakenly suggest.
Perhaps even better; compare the Pleiades with the Moon. The Moon appears to be larger than the whole cluster, but the Mooon can actually fit inside the 'bowl' of this miniature dipper.