http://tetsujinex.web.fc2.com/shop.html
his store
http://maniax.dlsite.com/work/=/product_id/RJ052043
the product in question
http://tetsujinex.web.fc2.com/sample6.html
sample images
Tha main story is about a fat virus, it causes the fat cells to multiply rapidly, there is no cure in the comic and girls infected gain massive amounts of energy.
The part with Cardcaptor weight gain is just a 4 page comic about Sakura finding a card of fat.