A damage deposit only covers minor damage. A landlord could sue you for the major damages you cause. Get some hidden camera video of your fatmate breaking floorboards so you can throw her under the bus during your trial.

I’d be complaining about the weak flooring rather than your big flatmate. A person would have to be verrry heavy to literally break good flooring just by walking on it

If your flooring can't support one fat person, what's it supposed to do if you have guests over? Your flooring is the problem, not your roommate--the floor should be able to hold all sorts of things, including a fat human. Get the landlord involved, because someone did a shit job setting your floors up.

I'd never expect to get a deposit back. Landlords are professionals in finding says to make sure you never do.

I hope you have renters insurance. if not get it.

Your roommate would have to be a literal elephant to do that. That's shitty architecture no matter how you slice it.