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 GRE_PLUGINS environment variable.
Set sbengine's 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 (i.e. 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.