Blatently obvious. If a fat person tries to sneak round a corner they will be seen, as their belly will protrude first and give them away. Hence if something can be seen a mile off, or is really obvious, it can be 'seen coming like a fat man round the corner'
Phil: 'Hey, did you hear Janice was fired?'
Butch: 'Yeah, I saw that coming like a fat man round the corner!"
a pointless-story-ending, similar to "and then i found ten dollars." when your story went nowhere, you put this on the end to make it interesting. generally used satirically to admit story-telling defeat.
cammie:well then it looked like the car had a scratch, but then it didn't really...