--- a/devtools/client/netmonitor/performance-statistics-view.js
+++ b/devtools/client/netmonitor/performance-statistics-view.js
@@ -205,17 +205,17 @@ PerformanceStatisticsView.prototype = {
         type = 3;
       } else {
         // "other"
         type = 9;
       }
 
       if (emptyCache || !responseIsFresh(details)) {
         data[type].time += details.totalTime || 0;
-        data[type].size += details.contentSize || 0;
+        data[type].size += details.transferredSize || 0;
       } else {
         data[type].cached++;
       }
       data[type].count++;
     }
 
     return data.filter(e => e.count > 0);
   },