diff --git a/test/js/web/fetch/fetch-leak.test.ts b/test/js/web/fetch/fetch-leak.test.ts
index 5c9a9cf290d1..6018e4b1a88e 100644
--- a/test/js/web/fetch/fetch-leak.test.ts
+++ b/test/js/web/fetch/fetch-leak.test.ts
@@ -673,7 +673,10 @@ describe("Request body HiveRef pool returns slot via Body.Value.deinit (does not
       kind,
       async () => {
         const script = `
-        const payload = Buffer.alloc(128 * 1024, 0x61); // 128 KiB of 'a'
+        // 16 KiB bodies: cycle() keeps 512 alive at once and gc() may not have returned that
+        // memory to the OS by the time rss() is sampled, so one cycle's live set (8 MiB here)
+        // is the noise floor of this measurement and must stay far below the threshold.
+        const payload = Buffer.alloc(16 * 1024, 0x61); // 16 KiB of 'a'
         const str = payload.toString("latin1");
         const sharedBlob = new Blob([payload]);
         const rss = process.platform === "darwin" && typeof Bun.unsafe.memoryFootprint === "function" ? Bun.unsafe.memoryFootprint : process.memoryUsage.rss;
@@ -705,18 +708,25 @@ describe("Request body HiveRef pool returns slot via Body.Value.deinit (does not
 
         const deltaMB = (final - baseline) / 1024 / 1024;
         console.log(JSON.stringify({ kind: "${kind}", baselineMB: (baseline / 1024 / 1024) | 0, finalMB: (final / 1024 / 1024) | 0, deltaMB: Math.round(deltaMB) }));
-        // 32 cycles * 512 Requests * 128 KiB = 2 GiB through the pool. If deinit() is
-        // skipped for any heap-backed variant, RSS climbs by ~2 GiB; with the
+        // 32 cycles * 512 Requests * 16 KiB = 256 MiB through the pool. If deinit() is
+        // skipped for any heap-backed variant, RSS climbs by ~256 MiB; with the
         // Zig semantics it stays flat. 64 MiB is well above GC/allocator noise.
-        // ASAN's quarantine retains freed allocations so widen the threshold there.
-        if (deltaMB > ${isASAN ? 320 : 64}) {
+        // The ASAN bound must also stay below that 256 MiB signal; the child's
+        // quarantine is capped (env below) so freed churn recycles, and 128 MiB
+        // covers what ASAN still retains (observed delta ~6 MiB).
+        if (deltaMB > ${isASAN ? 128 : 64}) {
           throw new Error("Request body (${kind}) leaked " + Math.round(deltaMB) + " MB over 32 cycles of 512 Requests");
         }
       `;
 
         await using proc = Bun.spawn({
           cmd: [bunExe(), "--smol", "-e", script],
-          env: bunEnv,
+          env: {
+            ...bunEnv,
+            // Cap the quarantine so it cannot retain freed bodies toward the
+            // default 256 MB, which would cross the 128 MiB leak threshold.
+            ...(isASAN && { ASAN_OPTIONS: `${bunEnv.ASAN_OPTIONS ?? ""}:quarantine_size_mb=32`.replace(/^:/, "") }),
+          },
           stdout: "pipe",
           stderr: "pipe",
         });
@@ -742,13 +752,19 @@ test("should not leak using readable stream", async () => {
     env: {
       ...bunEnv,
       SERVER_URL: server.url.href,
-      // ASAN's quarantine retains freed allocations so RSS stays elevated
-      // under bun-asan; the fixture only allows MAX_MEMORY_INCREASE MiB.
-      MAX_MEMORY_INCREASE: isASAN ? "64" : "5", // in MB
+      // The regression this guards retained the 128 KiB body per request, i.e.
+      // ~32 MiB over the 250 sampled iterations; RSS of a non-leaking run still
+      // drifts over that window (allocator high-water, lazily freed pages):
+      // a few MiB on Linux, up to 14 observed on Windows lanes. The allowance
+      // sits between that drift and the 32 MiB signal. Under bun-asan the
+      // capped quarantine (below) saturates before the sample point: clean
+      // runs measure 1-3 MiB, a simulated regression ~76, so 24 keeps this
+      // bound below the signal there too.
+      MAX_MEMORY_INCREASE: isASAN ? "24" : "20", // in MB
       // The fixture asserts RSS stabilizes after iteration 250, but with the
       // default 256 MB quarantine the freed 128 KB bodies are never reused and
       // RSS keeps climbing through all 500 iterations (~97 MB past the sample
-      // point, over the 64 MB allowance). Cap the quarantine so freed churn
+      // point, far over the allowance). Cap the quarantine so freed churn
       // recycles and the stabilization the test asserts can actually happen.
       ...(isASAN && { ASAN_OPTIONS: `${bunEnv.ASAN_OPTIONS ?? ""}:quarantine_size_mb=32`.replace(/^:/, "") }),
     },