diff --git a/scripts/build_installer.py b/scripts/build_installer.py
index 23c936c0..53ded458 100644
--- a/scripts/build_installer.py
+++ b/scripts/build_installer.py
@@ -9,6 +9,7 @@
 import platform
 import re
 import shutil
+import stat
 import subprocess
 import tempfile
 
@@ -71,6 +72,20 @@ def windows_installer(package: Path, output: Path, manifest: dict, compiler: str
     return asset
 
 
+def dmg_size_megabytes(layout: Path) -> int:
+    """Budget logical payload bytes, filesystem blocks and HFS+ metadata."""
+    payload = entries = 0
+    for path in layout.rglob("*"):
+        info = path.lstat()
+        entries += 1
+        if stat.S_ISREG(info.st_mode):
+            payload += ((info.st_size + 4095) // 4096) * 4096
+    # Source-folder auto-sizing can underestimate sparse/compressed binaries.
+    # Do not follow Applications or other bundle symlinks into the build host.
+    capacity = payload + payload // 5 + entries * 4096 + 32 * 1024 * 1024
+    return (capacity + 1024 * 1024 - 1) // (1024 * 1024)
+
+
 def macos_installer(package: Path, output: Path, manifest: dict, *, capture: bool = False) -> Path:
     if platform.system() != "Darwin":
         raise ValueError("DMG creation and verification must run on macOS.")
@@ -87,6 +102,7 @@ def macos_installer(package: Path, output: Path, manifest: dict, *, capture: boo
         (layout / "Applications").symlink_to("/Applications")
         shutil.copy2(Path(__file__).parents[1] / ("docs/capture-tool.md" if capture else "docs/install.md"), layout / "Install.txt")
         subprocess.run(["hdiutil", "create", "-volname", name, "-srcfolder", str(layout),
+                        "-fs", "HFS+", "-size", f"{dmg_size_megabytes(layout)}m",
                         "-format", "UDZO", "-ov", str(asset)], check=True)
         subprocess.run(["hdiutil", "verify", str(asset)], check=True)
         mounted.mkdir()
diff --git a/tests/test_macos_installer.py b/tests/test_macos_installer.py
new file mode 100644
index 00000000..505ace54
--- /dev/null
+++ b/tests/test_macos_installer.py
@@ -0,0 +1,79 @@
+from __future__ import annotations
+
+from pathlib import Path
+import shutil
+import sys
+import tempfile
+import unittest
+from unittest.mock import patch
+
+sys.path.insert(0, str(Path(__file__).parents[1] / "scripts"))
+import build_installer
+
+
+class MacInstallerTests(unittest.TestCase):
+    def test_sparse_payload_is_sized_from_full_length_with_filesystem_headroom(self):
+        with tempfile.TemporaryDirectory() as temporary:
+            layout = Path(temporary)
+            binary = layout / "native-helper"
+            with binary.open("wb") as stream:
+                stream.truncate(64 * 1024 * 1024 + 1)
+            size = build_installer.dmg_size_megabytes(layout) * 1024 * 1024
+            self.assertGreaterEqual(size, binary.stat().st_size + 32 * 1024 * 1024)
+
+    @unittest.skipIf(sys.platform == "win32", "Mac layout uses Unix symlinks")
+    def test_applications_link_does_not_add_the_hosts_applications_to_the_volume(self):
+        with tempfile.TemporaryDirectory() as temporary:
+            root = Path(temporary)
+            layout = root / "layout"
+            layout.mkdir()
+            outside = root / "host-applications"
+            outside.mkdir()
+            with (outside / "unrelated-app").open("wb") as stream:
+                stream.truncate(512 * 1024 * 1024)
+            before = build_installer.dmg_size_megabytes(layout)
+            (layout / "Applications").symlink_to(outside, target_is_directory=True)
+            self.assertLessEqual(build_installer.dmg_size_megabytes(layout), before + 1)
+
+    @unittest.skipIf(sys.platform == "win32", "Mac layout uses Unix symlinks")
+    def test_both_products_create_sized_images_and_verify_the_mounted_payload(self):
+        for capture in (False, True):
+            with self.subTest(capture=capture), tempfile.TemporaryDirectory() as temporary:
+                root = Path(temporary)
+                package, output = root / "package", root / "output"
+                output.mkdir()
+                name = "Focalet Capture" if capture else "Focalet"
+                app = package / f"{name}.app"
+                app.mkdir(parents=True)
+                (app / "binary").write_bytes(b"accepted signed payload")
+                calls = []
+                source = None
+
+                def run(arguments, *, check):
+                    nonlocal source
+                    self.assertTrue(check)
+                    calls.append(arguments)
+                    if arguments[0] == "ditto":
+                        shutil.copytree(arguments[1], arguments[2], symlinks=True)
+                    elif arguments[:2] == ["hdiutil", "create"]:
+                        source = Path(arguments[arguments.index("-srcfolder") + 1])
+                        self.assertEqual(arguments[arguments.index("-fs") + 1], "HFS+")
+                        capacity = arguments[arguments.index("-size") + 1]
+                        self.assertTrue(capacity.endswith("m"))
+                        self.assertGreaterEqual(int(capacity[:-1]), 32)
+                        Path(arguments[-1]).write_bytes(b"DMG fixture")
+                    elif arguments[:2] == ["hdiutil", "attach"]:
+                        mounted = Path(arguments[arguments.index("-mountpoint") + 1])
+                        shutil.copytree(source, mounted, symlinks=True, dirs_exist_ok=True)
+
+                with patch.object(build_installer.platform, "system", return_value="Darwin"), \
+                        patch.object(build_installer.subprocess, "run", side_effect=run):
+                    asset = build_installer.macos_installer(package, output, {"architecture": "arm64"}, capture=capture)
+                self.assertTrue(asset.is_file())
+                self.assertTrue(any(call[:2] == ["hdiutil", "verify"] for call in calls))
+                self.assertTrue(any(call[:2] == ["codesign", "--verify"] for call in calls))
+                self.assertEqual(calls[-1][:2], ["hdiutil", "detach"])
+
+
+if __name__ == "__main__":
+    unittest.main()