I am amazed at how much some people are trying to refute Midgarsorm as a boss here.
It is as they don't even read the storyline while playing, I don't see any other explanation.

So far, Mid is the only being that managed to damage Omega, even not clearly said, it is pretty obvious with how they interact that Mid cause the damages that made Omega to land on Hydaelin and fell into slumber.
It is so far the strongest foe the machine ever encountered, and it is no surprise we face a recreation of how Mid was when they faced each other right before fighting against omega.

As for the size argument, it is mentionned that dragons keep growing the older they get. Midgardsorm wraped around the Agrius is way older than when Omega met him. Allag was around 5000 years before when we are playing, and they found Omega already here, stating that Omega have faced Mid more than 5000 years ago.
Based on the trailer, he looks like being the size of Niddhogr and Hraesvelgr, so they may faced each other when mid was around the age of his children.

We will face the true strengh of midgardsorm, when he was younger and hasn't sacrified his power by bonding with hydaelin.