One of the great things about Photoshop is that it can be used to make people look thinner than they really are. This can be accomplished in a number of ways, but the most common is to use the Liquify filter. This filter can be found under the Filter menu, and then under Distort.
Once you have selected the Liquify filter, you will see a number of different options that you can use to thin out your subject. The first is to use the Forward Warp tool.
This tool is located in the upper left hand corner of the screen, and looks like a small arrow. To use this tool, simply click and drag it over the area of the image that you want to thin out. As you do this, you will see the area being pulled inwards.
Another way to thin out your subject is to use the Pucker tool. This tool is located just below the Forward Warp tool, and looks like a small circle.
To use this tool, simply click and drag it over the area of the image that you want to pucker. As you do this, you will see the area being pulled inwards and becoming smaller.
The last way to thin out your subject is to use the Bloat tool. This tool is located just below the Pucker tool, and looks like a small square.
To use this tool, simply click and drag it over the area of the image that you want to bloat. As you do this, you will see the area being pushed outwards and becoming larger.
All of these tools can be used to make your subject look thinner than they really are. Experiment with each one until you find the one that gives you the look that you want.