i think the question can be answered through another question.
'why is the bid-offer spread of IG index almost always more than HY??'
you can see their current levels at
http://www.markit.com/en/
you will see the reason is that IG is always quoted in bps while HY is always quoted as a price.
to be more elaborate when IG moves by 5 HY moves only by say 2.
the 5 is 5 bps on 92(bps) while 2 is 2 % on 101(price).
thus the volitility of IG would be almost always greater than HY because of a scale difference.