diff --git a/install.ps1 b/install.ps1
index 3aee95a74..fb834c753 100644
--- a/install.ps1
+++ b/install.ps1
@@ -53,8 +53,19 @@
                     "Check the network connection and any proxy, then run the same command again. " +
                     "Nothing was run and nothing was written.") -f $url, $_.Exception.Message
             }
-            if ((Get-Item -LiteralPath $temporary).Length -gt $maximumInstallerBytes) {
-                throw "Downloaded installer exceeds the size limit."
+            $downloaded = (Get-Item -LiteralPath $temporary).Length
+            if ($downloaded -gt $maximumInstallerBytes) {
+                # A page served in place of the file says the network
+                # answered, not this project: the one cause a user can act on.
+                $tooLarge = ("The downloaded installer is {0} bytes, more than the {1} this script accepts. " +
+                    "Nothing was run and nothing was written. A network that answers with a page of its own " +
+                    "(a sign-in portal, a proxy) gives this: open any web page to sign in, then run the same " +
+                    "command again.") -f $downloaded, $maximumInstallerBytes
+                if ([Environment]::GetEnvironmentVariable("RETROBIOS_INSTALL_URL")) {
+                    $tooLarge += " RETROBIOS_INSTALL_URL is set in this session and names another installer: " +
+                        "remove it to use the installer this script names."
+                }
+                throw $tooLarge
             }
             $actual = (Get-FileHash -LiteralPath $temporary -Algorithm SHA256).Hash.ToLowerInvariant()
             if ($actual -ne $expected.ToLowerInvariant()) {
diff --git a/install.sh b/install.sh
index 133806530..afb8f61e7 100755
--- a/install.sh
+++ b/install.sh
@@ -94,7 +94,16 @@ if [ -z "$INSTALLER" ] || [ ! -f "$INSTALLER" ]; then
   fi
   actual_size=$(wc -c < "$TEMP_INSTALLER" | tr -d ' ')
   if [ "$actual_size" -gt "$MAX_INSTALLER_BYTES" ]; then
-    echo "Error: downloaded installer exceeds the size limit." >&2
+    # A page served in place of the file says the network answered, not this
+    # project: the one cause a user can act on.
+    echo "Error: the downloaded installer is $actual_size bytes, more than the $MAX_INSTALLER_BYTES this script accepts." >&2
+    echo "  Nothing was run and nothing was written. A network that answers with a page of" >&2
+    echo "  its own (a sign-in portal, a proxy) gives this: open any web page to sign in," >&2
+    echo "  then run the same command again." >&2
+    if [ -n "${RETROBIOS_INSTALL_URL:-}" ]; then
+      echo "  RETROBIOS_INSTALL_URL is set in this environment and names another installer:" >&2
+      echo "  unset it to use the installer this script names." >&2
+    fi
     exit 1
   fi
   if command -v sha256sum >/dev/null 2>&1; then
diff --git a/tests/test_install.py b/tests/test_install.py
index 1f90be871..90008eab2 100644
--- a/tests/test_install.py
+++ b/tests/test_install.py
@@ -4764,6 +4764,38 @@ class TheBoardSystemsAreNamedFromTheCode(unittest.TestCase):
                 self.assertIn(name, paragraph)
 
 
+class TheInstallerFitsTheOneLiners(unittest.TestCase):
+    """install.sh and install.ps1 refuse an installer over 2 MiB, and
+    install.py grew from 596 KB to 1.1 MB in twelve days with nothing
+    holding it under that: past it, every one-line install would stop."""
+
+    def _caps(self) -> dict[str, int]:
+        import re
+
+        shell = (REPO_ROOT / "install.sh").read_text(encoding="utf-8")
+        ps1 = (REPO_ROOT / "install.ps1").read_text(encoding="utf-8")
+        units = {"": 1, "KB": 1024, "MB": 1024 ** 2}
+        number, unit = re.search(r"\$maximumInstallerBytes = (\d+)(KB|MB)?", ps1).groups()
+        return {"install.sh": int(re.search(r"^MAX_INSTALLER_BYTES=(\d+)$", shell, re.M)[1]),
+                "install.ps1": int(number) * units[unit or ""]}
+
+    def test_a_quarter_of_each_cap_stays_free(self):
+        size = (REPO_ROOT / "install.py").stat().st_size
+        for script, cap in self._caps().items():
+            with self.subTest(script=script):
+                self.assertLessEqual(
+                    size, cap * 3 // 4,
+                    f"install.py is {size} bytes, past three quarters of the {cap} {script} "
+                    "accepts: shrink it or raise the cap in both one-liners")
+
+    def test_the_refusal_names_a_cause_and_a_way_out(self):
+        for script in ("install.sh", "install.ps1"):
+            text = " ".join((REPO_ROOT / script).read_text(encoding="utf-8").split())
+            with self.subTest(script=script):
+                self.assertIn("this script accepts", text)
+                self.assertIn("open any web page to sign in", text)
+
+
 class RetroArchAppImageIsPortable(unittest.TestCase):
     """The buildbot's Linux RetroArch is an AppImage beside a .home folder
     the AppImage runtime makes its HOME: the run found nothing and offered