A simple script to automatically downsize all the images in a folder using Gimp python console (Gimp->Filters/Python-Fu/console).

gimp_img_downsize.py

from os import listdir
from os.path import isfile, join

## Set image folder path
dir_path = "C:/img_dir/"

## Store files names in folder
files = [f for f in listdir(dir_path) if isfile(join(dir_path, f))]

for file in files:
    image = pdb.file_jpeg_load(dir_path+file, dir_path+file)
    ## Don't forget to create the 'small/' folder in your image directory
    save_path = dir_path+"small/"+ file
    ## save image with 85% of original quality
    pdb.file_jpeg_save(image, image.active_drawable, save_path, save_path, 0.85, 0, 0, 0, "", 0, 0, 0, 0)