What you say when someone begins wholeheartedly and enthusiastically agreeing with your point of view, but you would have thought that this particular person would not truly be willing to go along with your idea, and so you suspect that he is merely trying to put you in a bettermood and is thus building up to saying, "But in spite of my sympathy for your position/situation, I still feel that..."
You: I feel that Klondike bars are the greatest thing since sliced bologna, so I think that every child should be allowed to eat at least one per day..
Parent: Oh, I agree with you --- they are super-delicious, fairly inexpensive, and an all-around ---
You: I sense a "but" coming here.
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!"