Darwinism turned on its head?

An article in the Independent describes how St Kilda's sheep are shrinking as an adaptation to changes in climate.  The article describes how animals restricted to islands can show interesting changes in size, with some being much bigger and some much smaller than their mainland equivalents.

So far so good.  All sound science.

But the subtitle of the article is:

"Darwinism turned on its head as milder winters allow smaller lambs to survive"

What is this supposed to mean?  Charles Darwin proposed that evolution can only make animals grow bigger?