Good question. First, each CVS is smaller than a Wag store. 11,000 sq ft as opposed to 14,500. Second, CVS employs a total of 105,000, Wag 154,000. I may be going way out on a limb here but I would assume CVS to be a more effecient operation. I would have expected a larger difference in sales. Perhaps thats why CVS has 86% institutional ownership of it's stock and Wag only 56%.
Part of the problem is your employee count is wrong. The latest figure I found for WAG's employee count is 93,000. It's probably higher than that now, but nowhere near 154,000. 105,000 seems to be the correct number for CVS.
Let me suggest that, instead of making assumptions (and you know where assumptions lead), why don't you try to research and find JUST ONE WAY that CVS is more efficient than WAG. Just one. I really mean it. You might actually find one, but I think you'll have to look pretty hard.