Mozz71, it really doesn't prove anything, It's something to do with the density of the sweetener used in regular Coke I came across this "fact" on a hot summer day when I put some Cokes in a stream to get cool...The diet Coke proceeded to float merrily down-stream:)
I hate artificial sweeteners. I can taste them a mile off. I found a great company that sell excellent squashes/cordials without them, along with 2 brands in the supermarket. Finding food stuff without sweetner in is getting harder and harder to come by and everything is tasting sweeter than it did a few years ago, especially savoury food.