The built-in gallery feature is a bit limited in functionality, to be honest. It only generates cached images for the thumbnails, but uses the original image files in the colorbox, which can be slow depending on the file size. I’ve found I often just use it as a basis for writing a custom gallery for a particular site. For example, to ensure it resizes each file, you would want to loop through the file list and pass them through the
Image::resize() method, found in
apps/filemanager/lib/Image.php. Let me know if that helps.