Motion's caching will utilize your computer's RAM, taking as much memory as defined in the Project Cache preference.
To view or change this setting in Motion, do the following:
The Project Cache setting represents the percentage of your computer's total memory that you wish to use. The larger the number, the more memory will be available to Motion to cache frames, resulting in a higher likelihood of consistently smooth playback.
The Project Cache setting is retained as global to Motion; when you close a project, the memory is still reserved while Motion is running. This memory reservation may affect the performance of other applications that are running at the same time as Motion.
Depending on your work style, there are a number of approaches to balancing the memory requirements of Motion and other applications that you run.