Interesting one.
Healthy - eating?
I'd hazard a guess at a few things. People are poorly educated regarding nutrition. Everything I ever learnt at secondary school I have long forgotten and I've had to 'relearn' in a recent effort to get my backside in gear. I'd propose that a lot of people simply don't know this stuff. Possibly hence the ideas with traffic light systems on food packs etc. We were taught it in Home Economics, no idea where kids are taught it now, if at all. I think we forget (and here I mean quite literally you and I) that we are well educated and intelligent, many others are not.
Then, beyond lack of education, lifestyles have changed so much. There are a number of factors. The old fashioned family concept is dated, people no longer cook meals and eat together. People don't have the time to buy fresh produce nor cook it (even though cooking food isn't really time consuming!) Convenience food is the norm and convenience food isn't good for you but it is... convenient. People are no longer as active as they were, so forget the sporty lot, I'm thinking more along the lines of jobs that involve manual labour (these days we're all spreading our arses driving desks), people don't walk so much to get to work or school or the shops. Then, additionally, people are working longer hours. So, they get up, drive to work, work too long, drive home, chuck something in the microping, watch a bit of telly and go to bed.
All of these things that have been invented to make our lives easier have just turned us into a bunch of fat, idle slobs like those blobby humans on Wall-E, remember? Furthermore, society has 'normalised' being larger, so a size 10 isn't a size 10 to us who remember what a size 10 used to be like, if you get my drift. Marks and Spencer resized a few years ago, and received public attention for doing so.
Fit - hmmmm. Harder one really. Society is not naturally as fit as it was, as we're not walking, riding bikes etc to work. We're driving. I have no idea why people don't want to get up and do stuff, because I do I guess? I suppose this is why we shouldn't take the mickey out of things like R4L like we do, because it is getting people into exercise.
So, in summary, I have provided several paragraphs of unsubstatiated rambling generalisations and poorly made points (with the exception of M&S resizing which I could probably google) but I think some of the rest may be true.
I do think humans will end up like the fat blobby ones on that film though, I really do.
I hope I don't get lambasted now. 
Nice topic KK.