Whats the prime motivation of the jersey? Strikes me that there are proably two 'issues'.
1. To change the attitude of other people to obese people (make them more supportive than condemming)
and/or
2. To motivate obese people to lose weight.
A cycling jersey is a relatively small canvas, so I suspect you need to tackle one message at a time - one message per jersey design. Make images bold, and any text short and punchy. Very few people will have time to read a complicated message on a jersey. Even pro jerseys with dozens of adverts on work because they conform to a design that is easily (nay, instantly) recognisable to followers.
If you're aiming the message at the wearer (in a self esteem boosting way), then you can perhaps allow for more detail, because the wearer knows what it says anyway.
I think for a prime example of designs that work fast, and make an impact, you should look at Foska's designs. See how they use big bold images, or images that are so ingrained into our psyche (like Marmite), that one only has to catch a glimpse to recognise them.
Maybe the first step is to think of a slogan - short and punchy, then work a design round it?
How about, if you don't mind the F word: I WAS FAT NOW I'M NOT printed on a jersey - first three words bigger than the second, something like:
I WAS
FAT
NOW I'M
NOT
or
NOW I'M
FAT
SOON I
WON'T BE
The key to both being that if you're wearing the jersey on a bike, then the clue as to 'why' the first and second statements differ is the very fact that you're on the bike. To really ram that home, stick a little cycle symbol between the two statements.
(Actually, that does help to tackle both issues, I guess, it shows obese people doing something about it, which improves the general level of support and sympathy)
Underneath, you'd have your web or blog address, for anyone who really wanted too take notice.