Man, I wonder how many of these threads I have seen on this board before.....
My view: Denmark is not really too weak, not much anyways, the decline is just quicker than historically. Sweden always breaks out far too early.
Trouble is, Denmark can sometimes expand, but they always explode soon after. Always. Nasty events, bad rulers and AI-trouble with disconnected isles - and especially the utter disregard of BB after Norway is inherited is the problem. In the hands of a player, Denmark is every bit as strong as it should be (was), compared to most other countries.
Sweden on the other hand is too strong, and almost always does rather well in SP. A poorly doing Sweden is only a bit smaller than in history. How often do you see Sweden disappear (except the rare vassal-anex)? Denmark does all the time. BTW, do Sweden have some kind of "die-hard" AI? I have once seen all of Sweden except two Finnish counties conquered by Denmark for a looong time, never giving up and signing peace (reloading didn't help). Denmark lost that war, couldn't take the WE.
German culture for Denmark: May help a bit, but I doubt it. Very strong tool in the hands of a player though. Historical? Perhaps the ruling class had a bit of both for a while, but that was probably true many other places......
And to Vorre: I believe a Prince or Count of Oldenburg "inherited" the Danish Throne in a way, so no conquest. As I remember, it was given away in an inheritance dispute again, not that long after - thus not lost in war.