diff --git a/wagtail/images/models.py b/wagtail/images/models.py
index 912011c0d5..5f71c3abcf 100644
--- a/wagtail/images/models.py
+++ b/wagtail/images/models.py
@@ -408,7 +408,7 @@ class Filter:
 
                 return willow.save_as_jpeg(output, quality=quality, progressive=True, optimize=True)
             elif output_format == 'png':
-                return willow.save_as_png(output)
+                return willow.save_as_png(output, optimize=True)
             elif output_format == 'gif':
                 return willow.save_as_gif(output)