Added paragraph about bloating and pytest

Author: astrofrog

peps/pep-0771.rst

@@ -663,6 +663,18 @@ installations and complicating dependency trees. Using default extras does not
 mean that all extras need to be defaults, and there is still scope for users to
 explicitly opt in to non-default extras.
 
+In essence, default extras should be considered just as 'weighty' as a required
+dependency. In some cases, if a package is widely used by many others, if it
+adds a default extra, then unless all downstream packages update their
+dependencies to specifically request a minimal installation, the defaults will
+often end up getting installed. A concrete example of this is that the `pytest
+<https://docs.pytest.org>`_ package has almost 1500 plugins at the time of
+writing, all of which depend on pytest. If pytest was to start using a default
+extra, it would need to consider this very carefully, as if the plugins are not
+updated, installing any plugin will result in the default extra being
+installed. This is not to say that it should not use default extras, but just
+that the decision should be considered and planned carefully.
+
 Inheriting from default extras
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^