What's up with the sizes, eh?

Okay, so I've been losing/gaining/losing/gaining weight for a couple of years, now. I have clothes from my skinny days still in my closet. I can't wear any of them, but they still take up space....taunting me. I am not prepared to toss anything in hopes of, eventually, getting back into some of the cutest things I've ever owned. I do toss things that are too big, though. I guess somewhere in my mind I think that if I keep "fat" clothes, then I am expecting to gain weight and wear them again - not gonna happen!
Recently, I've been fighting with a pair of pants that I bought oh so long ago and that have become so big that I can pull them off and on without unzipping or unbuttoning them. They drag on me and cause uncomfortable looks from passersby. These pants should be sent to a good home in the recycle bin. I just can't bring myself to give them up. Is it because they are green? Is it because I don't have many pairs that I can wear right now? Is it because I'm a pack rat? If I could figure this out, I wouldn't be typing, would I?
Today, fresh out of the shower and rummaging through the closet, I came across this pair of pants. I put them on! I didn't need to unzip or unbutton them. I wore them around the bathroom for all of 2 minutes when I could take it no longer. Off them came! So I'm thinking to myself, "Self," (that's what I call myself)"If these are REALLY so big, the next size down should fit well." So Self and I went in to find another green pair of pants one size smaller. They were quickly pulled off the shelf and pulled up the legs. They went on! Albeit, they were snug, but they went on! They aren't comfortable enough to wear as snug as they are, so they came back off rather quickly. I'm not wearing green today.
The question, though, is why do clothing manufacturers size clothes the way they do? I mean think about it. If a size 12 is too big, a 10 should fit, but a 10 is too tight. If a size 8 is too big, then a size 6 should fit, but a 6 is too tight. If a size 22 is too big, surely a 20 should fit, no? When the 6, 8, or even the 20 is too tight, but the next size up is too big, should not the clothing manufacturers start working on ODD numbers? Today, I wear a size 21. Not a 22! YAY! But still not a 20. Bummer! Either I wear the pants that are too big and risk looking like a teenage boy with his pants falling off, or I wear the pants that are too tight and risk looking, well, foolish, still.
Where am I going with this, I'm sure you are asking yourself. First, the green pants are going in the recycle bin today. Second, I'm going to need to get on the Wii Fit today so that I can get into the smaller green pants. That's all I've got for ya. I'm stuck in the "in-betweens" and all I can do is gain or lose and gaining is not an option.