Mobile platforms typically have small screens with relatively low resolutions compared to desktop systems.
If you are porting a desktop application to a mobile platform, fine details are likely to have little visual impact. This enables you to simplify graphics assets. You can make changes such as:
reducing the size and bit depth of textures
reducing geometry complexity
simplifying or removing effects that have little visible impact.
These changes reduce memory bandwidth usage. This enables higher performance.
For more information on simplification, see Use approximations to improve performance.