diff --git a/examples/protocols/mqtt/custom_outbox/sdkconfig.defaults b/examples/protocols/mqtt/custom_outbox/sdkconfig.defaults
index 385da31baf..11f90ceacf 100644
--- a/examples/protocols/mqtt/custom_outbox/sdkconfig.defaults
+++ b/examples/protocols/mqtt/custom_outbox/sdkconfig.defaults
@@ -1,2 +1,3 @@
 CONFIG_MQTT_CUSTOM_OUTBOX=y
 CONFIG_COMPILER_CXX_EXCEPTIONS=y
+CONFIG_COMPILER_OPTIMIZATION_SIZE=y