outerkay: Sure: LOW is smaller and in principle could grow more. And if LOW and HD were in different markets I could even agree with you. But HD and LOW are near-clones. As they move into the same geographic areas, they are competing to saturate the _same market_. LOW faces the same saturation point as HD if they both continue the game of chicken.
If this point seems abstract to you, let's correspond again at shakeout time.
Regarding HD's decline in share price ... if you have any actual money riding on this, you should be careful to distinguish the stock market from the home-improvement market. These two very different arenas share just one crucial element: lower price for equivalent value is better for the buyer.