Clothes sizes

I've been moaning lately about how I've lost two stone but don't fit into clothes a size smaller. I came to the conclusion that the bigger you are, the more weight it takes to drop a dress size, which I do believe, but I realised today, that the skirt I wear most often, I also have the identical skirt, one size smaller, still in the bag. As my original skirt is huge on me, I felt confident this one would fit, but I can't even squeeze into it, and the zip is nowhere near fastening. Incredibly it is 7 inches (18 cm) smaller in the waist! According to the Evans website (where the skirts came from) the difference in the sizes should only be one inch! I have another new skirt from there, two sizes smaller than the one thats too small and the difference in the waist is less than one inch. Its incredibly annoying as there are hardly any high street shops that cater for plus sized women,so we rely on mail order. I understand sizes vary from shop to shop, but to have such huge discrepancies within a store is ridiculous. I can't wait until I can buy clothes from a wide variety of shops.