--- a/toolkit/components/telemetry/TelemetrySend.jsm
+++ b/toolkit/components/telemetry/TelemetrySend.jsm
@@ -1206,35 +1206,35 @@ var TelemetrySendImpl = {
     let converter = Cc["@mozilla.org/intl/scriptableunicodeconverter"]
                     .createInstance(Ci.nsIScriptableUnicodeConverter);
     converter.charset = "UTF-8";
     let startTime = Utils.monotonicNow();
     let utf8Payload = converter.ConvertFromUnicode(JSON.stringify(networkPayload));
     utf8Payload += converter.Finish();
     Telemetry.getHistogramById("TELEMETRY_STRINGIFY").add(Utils.monotonicNow() - startTime);
 
+    let payloadStream = Cc["@mozilla.org/io/string-input-stream;1"]
+                        .createInstance(Ci.nsIStringInputStream);
+    startTime = Utils.monotonicNow();
+    payloadStream.data = gzipCompressString(utf8Payload);
+
     // Check the size and drop pings which are too big.
-    const pingSizeBytes = utf8Payload.length;
-    if (pingSizeBytes > TelemetryStorage.MAXIMUM_PING_SIZE) {
-      this._log.error("_doPing - submitted ping exceeds the size limit, size: " + pingSizeBytes);
+    const compressedPingSizeBytes = payloadStream.data.length;
+    if (compressedPingSizeBytes > TelemetryStorage.MAXIMUM_PING_SIZE) {
+      this._log.error("_doPing - submitted ping exceeds the size limit, size: " + compressedPingSizeBytes);
       Telemetry.getHistogramById("TELEMETRY_PING_SIZE_EXCEEDED_SEND").add();
       Telemetry.getHistogramById("TELEMETRY_DISCARDED_SEND_PINGS_SIZE_MB")
-               .add(Math.floor(pingSizeBytes / 1024 / 1024));
+               .add(Math.floor(compressedPingSizeBytes / 1024 / 1024));
       // We don't need to call |request.abort()| as it was not sent yet.
       this._pendingPingRequests.delete(id);
 
       TelemetryHealthPing.recordDiscardedPing(ping.type);
       return TelemetryStorage.removePendingPing(id);
     }
 
-    let payloadStream = Cc["@mozilla.org/io/string-input-stream;1"]
-                        .createInstance(Ci.nsIStringInputStream);
-    startTime = Utils.monotonicNow();
-    payloadStream.data = gzipCompressString(utf8Payload);
-
     const compressedPingSizeKB = Math.floor(payloadStream.data.length / 1024);
     Telemetry.getHistogramById("TELEMETRY_COMPRESS").add(Utils.monotonicNow() - startTime);
     request.sendInputStream(payloadStream);
 
     return deferred.promise;
   },
 
   /**

--- a/toolkit/components/telemetry/tests/unit/test_TelemetrySend.js
+++ b/toolkit/components/telemetry/tests/unit/test_TelemetrySend.js
@@ -330,17 +330,17 @@ add_task(async function test_discardBigP
   let histSuccess = Telemetry.getHistogramById("TELEMETRY_SUCCESS");
   let histSendTimeSuccess = Telemetry.getHistogramById("TELEMETRY_SEND_SUCCESS");
   let histSendTimeFail = Telemetry.getHistogramById("TELEMETRY_SEND_FAILURE");
   for (let h of [histSizeExceeded, histDiscardedSize, histSuccess, histSendTimeSuccess, histSendTimeFail]) {
     h.clear();
   }
 
   // Generate a 2MB string and create an oversized payload.
-  const OVERSIZED_PAYLOAD = {"data": generateRandomString(2 * 1024 * 1024)};
+  const OVERSIZED_PAYLOAD = {"data": generateRandomString(4 * 1024 * 1024)};
 
   // Submit a ping of a normal size and check that we don't count it in the histogram.
   await TelemetryController.submitExternalPing(TEST_PING_TYPE, { test: "test" });
   await TelemetrySend.testWaitOnOutgoingPings();
   await PingServer.promiseNextPing();
 
   Assert.equal(histSizeExceeded.snapshot().sum, 0, "Telemetry must report no oversized ping submitted.");
   Assert.equal(histDiscardedSize.snapshot().sum, 0, "Telemetry must report no oversized pings.");