Don't upscale if the image is smaller than the size

src/PIL/Image.py

@@ -2236,18 +2236,18 @@ def thumbnail(self, size, resample=BICUBIC, reducing_gap=2.0):
         :returns: None
         """
 
-        # preserve aspect ratio
         x, y = size
+        if x >= self.width and y >= self.height:
+            return
+
+        # preserve aspect ratio
         aspect = self.width / self.height
         if x / y >= aspect:
             x = max(y * aspect, 1)
         else:
             y = max(x / aspect, 1)
         size = (round(x), round(y))
 
-        if size == self.size:
-            return
-
         box = None
         if reducing_gap is not None:
             res = self.draft(None, (size[0] * reducing_gap, size[1] * reducing_gap))