Being cruel to people is cruel. You can accomplish that by telling the truth too ("Yes you are fat", "Santa does not exist").
Lying is a tool, it can be used for good or bad.
Lying is a tool, it can be used for good or bad.
That's precisely the point. Here lying was done for personal gains.
Why are your examples cruel?
Someone learns that they need to address their weight problems before it affects their health badly or if they want to woo you or whatever.
Someone else learns that they receive presents not because they've been good but because people regard them; I was going to say love instead of regard but that's not always true.
Sure you can say things in cruel ways ...