There are a few batch processing plugins for GIMP that offer both absolute and relative resizing (relative being set in percentage to the original width and height). BIMP is one of them (binaries for Windows only, but source code is also available for compiling in Linux). Set W and H to, say, 60% and it will resize all your images and preserve aspect ratio no matter what sizes they are. In your example 700x200 image, set percentage to 10% and the resulting image is 70x20.
Few other GIMP plugins that can do the same - David's batch processor, Script-Fu Batch-Resize-and-Save-Image. The latter requires Liquid Rescale plugin and can do liquid rescale batch resizing, but before using it first you have to grasp the idea of how liquid rescale works to take full advantage of its capabilities. My recommendation - use BIMP, because it has some other useful features and the plugin developer is keeping up a good work regularly updating it and adding new functions.
P.S. If none of these satisfied you, install standalone application Phatch (available for Linux, Win & MacOS).