Many people are tricked into thinking that just because the number on the scale is lower, that they are healthier.
This is not always true.
Just as the picture shows, muscle weighs more than fat. As a result, particularly muscular people may think that they are overweight and/or unhealthy simply because of a number on a scale.
Here's an example of this principle. Imagine that I stopped training for a month (Impossible, I know.). I would probably lose at least 10 pounds, but I wouldn't be as healthy as I am normally. I would be losing fat and muscle.
Another way to gauge your healthiness is simply thinking about how you feel while doing certain exercises. Excess fat can make certain exercises in our workout feel more difficult than they would be otherwise (stretches, jumping jacks, etc.) A karate-ka in fighting shape should also be able to run at least a mile with relative ease.
In conclusion, if a martial artist says that they want to lose weight, hopefully they actually want to lose fat.