MongoDB $unset: "Executing this update would put you over your space quota"

Question

I have a collection containing Reddit comments, gathered by a bot. The schema looks like this:

{
    _id: ObjectId,
    author: string,
    body: string,
    permalink: string,
    created_utc: number,
    upvotes: number,
    downvotes: number
}

I would like to remove the upvotes and downvotes fields. I have tried using the following updateMany query:

use DBname

db.comments.updateMany({}, {$unset: {upvotes: "", downvotes: ""}})

Running this, I get the following error:

MongoServerError: Executing this update would put you over your space quota

This data base is located on MongoDB Atlas, in a free instance (limited at 512 MB). However I'm currently only using around 255 MB. I would expect this query to free up space, not to occupy more.

I should mention that these parameters do work with db.updateOne(), however I would like to run the query using a single operation, rather than writing a script and calling multiple updateOne()s.