diff --git a/crates/format/src/large_tests.rs b/crates/format/src/large_tests.rs
index 3ff352ac..2dc9eb4c 100644
--- a/crates/format/src/large_tests.rs
+++ b/crates/format/src/large_tests.rs
@@ -202,3 +202,18 @@ fn the_header_is_read_on_its_own() {
     let (_, _, index) = parse_json(&d.replacen("\"imageIndex\":[]", "\"imageIndex\":[{\"hash\":\"ab\"},{\"nope\":1}]", 1)).unwrap();
     assert_eq!(index, Some(["ab".to_string()].into_iter().collect()));
 }
+
+#[test]
+fn document_json_may_unpack_in_proportion_to_its_packed_size() {
+    const MB: u64 = 1024 * 1024;
+    // A small archive keeps the member cap: it can't unpack into gigabytes.
+    assert_eq!(document_limit(0, 0), MAX_MEMBER);
+    assert_eq!(document_limit(MB, 2 * MB), MAX_MEMBER);
+    // A real 900 000-layer document saved by us packs 1 477 139 810 bytes into 244 362 923, and
+    // didn't open again under the 512 MB cap.
+    assert!(document_limit(244_362_923, 260 * MB) > 1_477_139_810);
+    // The packed size an archive claims counts only up to the archive's own size.
+    assert_eq!(document_limit(u64::MAX, MB), MAX_MEMBER);
+    // Never more than the ceiling.
+    assert_eq!(document_limit(u64::MAX, u64::MAX), MAX_DOCUMENT);
+}
diff --git a/crates/format/src/lib.rs b/crates/format/src/lib.rs
index f88416e2..dd96b932 100644
--- a/crates/format/src/lib.rs
+++ b/crates/format/src/lib.rs
@@ -39,6 +39,11 @@ pub const FORMAT: &str = "figcraft";
 pub const VERSION: u32 = 1;
 /// Largest archive member we read (guards against zip bombs).
 const MAX_MEMBER: u64 = 512 * 1024 * 1024;
+/// `document.json` may unpack to [`DOCUMENT_RATIO`] times what it takes in the archive, within
+/// [`MAX_MEMBER`] and [`MAX_DOCUMENT`]: a large document saved by us (a 900 000-layer file packs
+/// 1.48 GB into 244 MB) opens again, while a small archive still can't unpack into gigabytes.
+const DOCUMENT_RATIO: u64 = 16;
+const MAX_DOCUMENT: u64 = 4 * 1024 * 1024 * 1024;
 /// Most versions a file keeps: saving more is refused, extra entries in a file are skipped.
 pub const MAX_VERSIONS: usize = 100;
 /// Longest version name and description kept (chars).
@@ -455,6 +460,11 @@ pub(crate) fn read_member<R: Read + Seek>(z: &mut zip::ZipArchive<R>, name: &str
     Ok(data)
 }
 
+/// Largest `document.json` read from an archive of `len` bytes in which it takes `packed`.
+pub(crate) fn document_limit(packed: u64, len: u64) -> u64 {
+    packed.min(len).saturating_mul(DOCUMENT_RATIO).clamp(MAX_MEMBER, MAX_DOCUMENT)
+}
+
 /// How many images, and how many bytes of them together, one file may give.
 #[derive(Clone, Copy, Debug)]
 pub(crate) struct ImageBudget {
@@ -476,7 +486,9 @@ pub(crate) fn load_within(bytes: &[u8], budget: ImageBudget) -> Result<Loaded, F
         return Ok(Loaded { issues, ..Loaded::from(doc) });
     }
     let mut z = zip::ZipArchive::new(std::io::Cursor::new(bytes)).map_err(|e| FormatError::Corrupt(e.to_string()))?;
-    let text = String::from_utf8(read_member(&mut z, "document.json", MAX_MEMBER)?).map_err(|e| FormatError::Corrupt(e.to_string()))?;
+    let packed = z.by_name("document.json").map(|f| f.compressed_size()).unwrap_or(0);
+    let limit = document_limit(packed, bytes.len() as u64);
+    let text = String::from_utf8(read_member(&mut z, "document.json", limit)?).map_err(|e| FormatError::Corrupt(e.to_string()))?;
     let (mut doc, mut issues, own_images) = parse_json(&text)?;
     let (images, unread) = read_images(&mut z, own_images.as_ref(), budget);
     report_lost_images(&mut issues, &unread, own_images.as_ref(), &images);