Is dieting different from losing weight and changing your body shape?

In the past, I thought that if the number on the scale went down, my diet was going well.

However, these days, even with the same weight
I often hear that my body shape looks quite different depending on my muscle mass or body fat.

As I get older, I also think that maintaining my body shape is more important than simply losing weight.
I want to know if I actually need to change my approach to my diet.