When something is getting smaller and smaller, or worse and worse, you can say that it is "on the decline".
Topics that you can use "on the decline" to talk about include:
- a person's health or athletic ability
- the economy
- crime statistics
I'm getting older now and my health is on the decline.
The simple truth is that this once-great nation is on the decline.
The murder rate is on the decline, and has been for the last decade.