Python PIL (Pillow) resizes my pictures after modifying exif data

i made a small script in Python, which can set the exif data of my old Whatsapp pictures based on their filename.

I use the piexif and the PIL (Pillow) package.

import piexif
from PIL import Image
from collections import defaultdict

img = Image.open(fname)

try:
    exif_dict = piexif.load(img.info["exif"])
except KeyError:
    exif_dict = defaultdict(dict)

exif_dict['Exif'][piexif.ExifIFD.DateTimeOriginal] = exiftime(date)
exif_dict['Exif'][piexif.ExifIFD.DateTimeDigitized] = exiftime(date)

exif_bytes = piexif.dump(exif_dict)
img.save('%s' % fname, "jpeg", exif=exif_bytes)

The exiftime() function is only for formatting the date.

However, the script is setting some exif fields, i don’t modify compression or someting like that.

My problem is, that the pictures get much smaller, after running that script.
I tested this script with some sample images, e.g. a picture shot with a Nikon D5300 with an resolution of 6000×4000. The original file has about 12Mb, after the script it has only 4Mb.

Does the script cause a quality loss of the picture, or is it just a better compression?