As you didn't get an answer, a quick one from me:

1. Roman, Anglo-Saxon, Viking, and Norman invasions forced the 'Britons' back into Wales (and Cornwall, and Scotland) because they were easier to defend in guerilla combat. So the Welsh population might have been artificially high. Britain was never properly invaded after the Norman conquest so that never repeated.
2. The Norman Conquest, and particularly Edward I's conquest of Wales were very bloody. I know we (the Welsh) lost 2,000 - 5,000 in one battle alone which would have been about 2% of the Welsh male population at the time.
3. Greater migration. It's a volatile time prior to the Norman conquest so people were attempting to increase their sphere of influence. There's significant evidence of quite a lot of Irish migration to Wales, and possibly some Scottish too.
4. Welsh emigration after the Normans/Edward I. As England's grip holds firmer, some Welsh people flee to Ireland and other 'Celtic' countries. Others emigrate to England who - while not incredibly rich - has many more affluent cities than Wales.
5. Weather changes. Not so convinced on this, but around about the Norman conquest, Wales was in an unusually warm spell. The differences between English and Welsh climate aren't huge, but Wales bears the brunt of storms coming from the west (the Atlantic).. The little ice age from 1300-ish onward, caused huge storms in Europe which could have plausibly affected Wales worse.