MPlay starts with a very large image cache. As images are loaded off disk, MPlay caches them in memory.
To have MPlay attempt to cache all the images in all loaded sequences, choose Image ▸ Cache All. This will adjust the cache size to fit all the images, if possible. The process will stop if the cache exceeds available memory.
To explicitly set the cache size, choose Windows ▸ Display Options and click the Memory tab.