The Cleveland Show website gives three possible answers:
http://theclevelandshowblog.com/2009/09/cleveland-jr-got-fat/
I would assume it was a combination of all three.
According to the Family Guy wiki, in a deleted scene it is revealed that he gains the weight because of the Ritalin he began taking for his ADHD.
See the full explanation here: http://familyguy.wikia.com/wiki/Cleveland_Brown,_Jr.
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It was explained as Cleveland Jr simply going through puberty.
However, it was revealed on The Cleveland Show that "Junior" actually killed Cleveland Jr and took his place due to the fact Junior is a spy and he was trying to kill Tim the Bear. Natually though this may have simply been a story Junior told Rallo to mess with him.
No. The Cleveland Show was never going to do any Star Wars parodies. At the time The Cleveland Show was on the air Lucasfilms was sold to Disney so that ended any references to Star Wars from Family Guy & The Cleveland Show because they couldn't get permission to do parodies.
The show did not get good enough ratings to cover the networks' cost of airing it, so it was cancelled.
No. Check his new dating show. Maybe he's just so pretty he doesn't know what to do with himself.
Because that's what a spinoff is, when a character from a show leaves to appear in his own show. If you mean why did he within the Family Guy world, he divorced Loretta and wanted a new start.
Bill Cosby, the creator. Had a group of friends he hung out with as a teenager, that group was later created into the show Fat Albert. The guy who was the actual Fat Albert past away, so they made the movie and dedicated it to him. At the ending seen it shows the actual group.