diff --git a/src/calibre/ebooks/metadata/mobi.py b/src/calibre/ebooks/metadata/mobi.py
index 408bab828d..30668d70f7 100644
--- a/src/calibre/ebooks/metadata/mobi.py
+++ b/src/calibre/ebooks/metadata/mobi.py
@@ -404,14 +404,16 @@ class MetadataUpdater(object):
                 if self.cover_record is not None:
                     size = len(self.cover_record)
                     cover = rescale_image(data, size)
-                    cover += '\0' * (size - len(cover))
-                    self.cover_record[:] = cover
+                    if len(cover) <= size:
+                        cover += '\0' * (size - len(cover))
+                        self.cover_record[:] = cover
                 if self.thumbnail_record is not None:
                     size = len(self.thumbnail_record)
                     thumbnail = rescale_image(data, size, dimen=MAX_THUMB_DIMEN)
-                    thumbnail += '\0' * (size - len(thumbnail))
-                    self.thumbnail_record[:] = thumbnail
-        return
+                    if len(thumbnail) <= size:
+                        thumbnail += '\0' * (size - len(thumbnail))
+                        self.thumbnail_record[:] = thumbnail
+                return
 
 def set_metadata(stream, mi):
     mu = MetadataUpdater(stream)
diff --git a/src/calibre/ebooks/mobi/writer.py b/src/calibre/ebooks/mobi/writer.py
index e8fc4557fd..5e4dca4a9e 100644
--- a/src/calibre/ebooks/mobi/writer.py
+++ b/src/calibre/ebooks/mobi/writer.py
@@ -112,15 +112,34 @@ def align_block(raw, multiple=4, pad='\0'):
 
 def rescale_image(data, maxsizeb, dimen=None):
     if dimen is not None:
-        return thumbnail(data, width=dimen, height=dimen)[-1]
-    # Replace transparent pixels with white pixels and convert to JPEG
-    data = save_cover_data_to(data, 'img.jpg', return_data=True)
+        data = thumbnail(data, width=dimen, height=dimen)[-1]
+    else:
+        # Replace transparent pixels with white pixels and convert to JPEG
+        data = save_cover_data_to(data, 'img.jpg', return_data=True)
+    if len(data) <= maxsizeb:
+        return data
+    orig_data = data
+    img = Image()
+    quality = 95
+
+    if hasattr(img, 'set_compression_quality'):
+        img.load(data)
+        while len(data) >= maxsizeb and quality >= 10:
+            quality -= 5
+            img.set_compression_quality(quality)
+            data = img.export('jpg')
+        if len(data) <= maxsizeb:
+            return data
+        orig_data = data
+
     scale = 0.9
     while len(data) >= maxsizeb and scale >= 0.05:
         img = Image()
-        img.load(data)
+        img.load(orig_data)
         w, h = img.size
         img.size = (int(scale*w), int(scale*h))
+        if hasattr(img, 'set_compression_quality'):
+            img.set_compression_quality(quality)
         data = img.export('jpg')
         scale -= 0.05
     return data
diff --git a/src/calibre/utils/magick/draw.py b/src/calibre/utils/magick/draw.py
index 80fd683196..88f488cb23 100644
--- a/src/calibre/utils/magick/draw.py
+++ b/src/calibre/utils/magick/draw.py
@@ -72,6 +72,8 @@ def thumbnail(data, width=120, height=120, bgcolor='#ffffff', fmt='jpg'):
         img.size = (nwidth, nheight)
     canvas = create_canvas(img.size[0], img.size[1], bgcolor)
     canvas.compose(img)
+    if fmt == 'jpg' and hasattr(canvas, 'set_compression_quality'):
+        canvas.set_compression_quality(70)
     return (canvas.size[0], canvas.size[1], canvas.export(fmt))
 
 def identify_data(data):