They were serialising the previous object as a string (which may also contain a key called previous which is a string) so quoted values, backslashes and any other escape characters would be escaped. Over multiple screens this grows quickly, e.g. this simple struct which has only an id column grows to 2KB in only 9 steps:

    let obj = { id: 1 }

    for (let id = 2; id <= 10; id++) {
      obj = { id, previous: JSON.stringify(obj) }
    }