The general theory is that for a racing boat that has a fixed max headsail dimension, a "Light" No. 1 would have a fuller draft and lighter material than the "Heavy" No. 1. The "Light" No. 1 would be a more powerful sail, fuller draft, yet not as high a pointing sail, enabling you to build speed and power through the chop that would slow a boat in light winds. In practice, the "Light" would go up to about 7 - 9 kts AWS and the "Heavy" would take it from there, to the No. 2's range, typically in the low to mid teens AWS. There really is no such thing as a "Light" No. 2 as that sail is optomized for higher wind speeds, and the next gear shift would be to the No. 3.
In addition, while feasable, you could have a "Light" mainsail and a "Heavy" mainsail, both with optomized drafts and materials for their expected wind ranges. I have sailed on boats, years ago, that had them, but sail count limits make them useless.
In regards to the comments about "Bloopers" helping to aleviate the IOR boats unstable downwind tendancies, I don't think so much... The Blooper was a VERY unstable sail, and never used in the highwind conditions that would create the combination of factors that would facilitate the famous IOR "Death Roll" or broach. They were exclusively used in light to moderate wind conditions, dead down wind running, and the proper technique actually had you LOWER the mainsail to gain the greatest benifits. Glad to see then go!
With the introduction of modern fractional rigged high performance boats, the mainsail was no longer relegated to a secondary role. The need for "Light" and "Heavy" headsails of the same size became secondary to the proper trim of the now larger and more powerful mainsails.