You shouldn't tell people to lose weight since it's rude.
John is at work when he sees Tim, an old teammate that was in another division for the past year who has gained significant weight.
John wants to comment on Tim's changed appearance.
John tells Tim that it's been so long since they've seen each other he hardly recognized Tim.
John tells Tim that he looks way bigger than the last time he saw him and that he needs to slim back down.
Tim happily replies that it's been a long time, indeed and that he's happy to see John.
Tim is insulted by the comment and now feels self conscious about his appearance.