I think the general rule is 15 lbs. for a size. You'll notice a bigger difference in size with a smaller weight loss when you are putting on muscle. So, if you've really changed your activity level, like gone from doing nothing to walking everyday you might see more of a loss in size that the number on the scale. Also, it depends on the cut of your clothing, as Paint mentioned somethings fit differently, so maybe something high-waisted might not feel as loose as low-waisted because you lost more size in your hips than waist.
A pound of muscle takes up less space than a pound of fat. I think somewhere on this site there's a picture of that. I'm sure you've heard that saying that muscle weighs more than fat, but a pounds a pound the world around, it doesn't weigh less it takes up less space. That's why you're toning up or shrinking when you start exercising.
I'm certainly no expert (except in having lost the same weight multiple times
) I try taking my measurements every so often because sometimes that gets you through a time when the scale doesn't budge, or is moving very slowly.