The Dragon, wounded, damn near killed him. It was a rough fight, not a walkover.
The Dragon splits Big E's armour open with one swipe, his leg pouring blood like a stream. The E's lance does jack to it, and his sword just bounces off. He gets dealt some fearful blows (mostly blocked) and has his helmet ripped from his head by a lightning bolt, still having done jack to the Dragon.
The Big E only wins because he spots a weak point to strike (possibly the wound from the Blackstone Fortress? I'm not up on that fluff yet) and strikes there, shutting it down temporarily.
‘The Dragon is defeated!’ cried the warrior. ‘But it is beyond
even my power to destroy, so I shall drag it in fetters from this
place and bind it deep in the darkness, where it will remain until
the end of all things.’'
The Emperor is not powerful enough to destroy the Dragon. He said so himself. (
pg 315 Mechanicum)
The Dragon got beat down, then imprisoned in stasis while he was down. You try getting better when you're stuck in a stasis field

As physics and
40K have little to do with each other, it's possible, but it takes a while..
That's assuming that it's not reasonably happy where it is for now, until things change or it does find a way to recover.
I would put them as being about on par for power, and if thr Dragon hadn't had the weak spot/wound to contend with, or the Big E hadn't spotted it, he would be quite dead