By default sbengine uses as much memory as it requires to load all the assets that the application requires (images, fonts, scripts,...) but this can be tuned to save memory. Here are some options to help with this.
Remove any unused plugins from the plugins directory if you are simply setting a directory for the SB_PLUGINS environment variable. The plugins that are available and being loaded will be shown by passing the -i option to the sbengine command line utility.
Set the resource_mgr options for image and font cache to appropriate values. Remember the caches must be large enough to fit all the images and fonts for your most resource intensive screen.
Use the Load Scaled flag in image render extension options if you are loading a scaled version of an image (e.g., an image thumbnails screen). If you are only ever loading the image once you should resize the image before deployment to avoid the runtime cost of image scaling.