Each stream in the archive has an associated <stream id>.toml that has the following fields:

pub struct StreamConfig {
    pub name: Option<String>,
    pub source_path: String,
    pub pack_time: toml::value::Datetime,
    pub size: u64,
    pub mapped_size: u64,
    pub packed_size: u64,
    pub thin_id: Option<u32>,
}

A simple example:

use serde_derive::{Deserialize, Serialize};

#[derive(Deserialize, Serialize, PartialEq)]
struct Config {
    big_num: u64,
}


fn main() {
    let cfg_start =  Config {big_num: u64::MAX};
    let string_rep = toml::to_string(&cfg_start).unwrap();
    let cfg_end = toml::from_str(&string_rep).unwrap();
    assert!(cfg_start == cfg_end);
}

The problem is the toml code expresses numeric values as i64. If you try to read back a toml file that has values which exceed 2**63-1 you'll get something like the following:

Error { inner: ErrorInner { kind: NumberInvalid, line: Some(0), col: 10, at: Some(10), message: "", key: [] } }
This is when you're using toml version "0.5.8".

If you use the latest version of toml, you get the following error when you try to serialize:
Error { inner: OutOfRange(Some("u64")) }