Smaller bags cost A LOT more. And they aren't air tight. I have had small pound sized unopened plastic bags from the grocery that went ROCK hard after only a few months. I know all the tricks to getting hard sugar soft again. Apple slices, bread slices, etc, I've done them all and they all work. However they all take planning ahead. In other words, if I got a hankering for some cookies, I want them today. Not tomorrow after the sugar has time to soften. Yea, I like immediate gratification.
So rather than go through the trouble of having the deal with softening it, I want to keep it from getting hard in the first place.

Of course all of that is moot anyway because that doesn't tell me if the 6 gallons I stored are safe to eat. I will revise my future storage methods as necessary.

Grey - the zip top mylar are awesome. You still have to heat seal them for storage, but they give you a handy re-sealable bag after it's been opened. I buy them from www.usaemergencysupply.com. those folks have always treated me right. Once or twice they have run out of stock of something I ordered and they have called me up to make sure it's ok if they instead send me a different, and more expensive, size for no charge so I don't have to wait for backorders.