Migraine is more common in MSers than non-MSers, but isn't officially related. Given that macropsia sometimes happens in migraine, I guess it would also be more common in MSers than in non-MSers, but I don't know if it's a sign of MS on its own.
I've just had a quick scan of wikipedia (and therefore know next to nothing about the subject!!!). It says that macropsia is when objects in the visual field are enlarged, i.e. the patient actually sees things as bigger than they are. You're only getting it when you have your eyes closed?
The following is complete speculation!:
If it is caused by distorted retinal signals then I guess your ON could be to blame, but I'm not sure how if it is how you feel and not what you are seeing (for real or in your "mind's eye").
If it is only your body parts that feel wrong, then perhaps there is something wrong in the bit of your brain that represents our body parts and by opening your eyes, reality "resets" things. The brain contains a sort of map of our bodies (in the somatosensory cortex, if my memory is right, and maybe in the motor cortex too?) where neurons are sorted into a replica of our bodies - so the neurons for the left hand are right next to the neurons for the left wrist, left arm, left shoulder, etc. The map isn't life-like though. Instead, the amount of space dedicated to each body part depends on how many neurons it needs (so hands and lips need loads, but the torso doesn't). It's illustrated by an "homunculus". If you have a google you'll see that this is a figure with weirdly sized body parts. So this is how our brain thinks of our bodies in sensory and motor terms.
Anyway, perhaps (somehow!) your brain's sensory map is overriding your knowledge of what your body actually is?
Not sure if this helps at all!
And after all that (and just to confuse things even more!), it could be migraine you know. Migraine can occur without headache.
At least your neuro should be interested - it can't be all that common a symptom!