“Everybody has a secret world inside of them. All of the people of the world, I mean everybody. No matter how dull and boring they are on the outside, inside
them they've all got unimaginable, magnificent, wonderful, stupid, amazing worlds. Not just one world. Hundreds of them. Thousands maybe.” -Neil Gaiman
An anorexic thinks she's fat even when she's underweight. What some people think is "fat" is curvy to another.
I mean telling someone they're medically obese is objective, but the generic term "fat" is often subjective.
Also, telling someone they won't amount to anything is subjective because different people have different strengths and different ideas of what would be good to accomplish in their life.
What you said actually sounds like verbal abuse, not truth.
Truth would be "You're not conventionally pretty, you're medically overweight, and if you don't get off your ass and stop smoking pot in your mom's basement, you may never do anything productive with your life."