Hi there! Because the forum you originally posted in is for beginners trying to learn the basics of Photoshop, I moved your question to the Photoshop Touch forum, where you'll get more specialized help.
There are several ways to make some one appear thinner using Photoshop Touch. One of my prefered methods is using the Warp tool.
1. Using the photo below as an example, I am going to try and make the female in the photo thinner.
2. Add another layer to make the adjustments. tap the Add layer Button > Duplicate Layer
3. With the new layer highlighted, tap & > Warp.
4. With the warp tool, adjust the warp mesh to your liking. I moved the handles inward, to make the female "skinny". Tap the check mark when you are done.
5. Tap the eraser tool to erase the man and help blend the two layers.
6. Below you can see as I am erasing to reveal the original layer below.
7. You can see the "before" and "after" below.
Please let me know if you have any questions. The Warp tool is a effective way to reduce size and affect the shape, and minimizes a "stretched" or "squeezed" look.