diff --git a/cmake/configs/nuttx_px4fmu-v2_default.cmake b/cmake/configs/nuttx_px4fmu-v2_default.cmake
index a5cee0ba13..5f495cdf7d 100644
--- a/cmake/configs/nuttx_px4fmu-v2_default.cmake
+++ b/cmake/configs/nuttx_px4fmu-v2_default.cmake
@@ -201,9 +201,11 @@ set(config_io_extra_libs
 add_custom_target(sercon)
 set_target_properties(sercon PROPERTIES
 	PRIORITY "SCHED_PRIORITY_DEFAULT"
-	MAIN "sercon" STACK_MAIN "2048")
+	MAIN "sercon" STACK_MAIN "2048"
+	COMPILE_FLAGS "-Os")
 
 add_custom_target(serdis)
 set_target_properties(serdis PROPERTIES
 	PRIORITY "SCHED_PRIORITY_DEFAULT"
-	MAIN "serdis" STACK_MAIN "2048")
+	MAIN "serdis" STACK_MAIN "2048"
+	COMPILE_FLAGS "-Os")
diff --git a/cmake/configs/nuttx_px4fmu-v4_default.cmake b/cmake/configs/nuttx_px4fmu-v4_default.cmake
index 452929a7ce..6dd6833119 100644
--- a/cmake/configs/nuttx_px4fmu-v4_default.cmake
+++ b/cmake/configs/nuttx_px4fmu-v4_default.cmake
@@ -204,10 +204,12 @@ add_custom_target(sercon)
 set_target_properties(sercon PROPERTIES
 	PRIORITY "SCHED_PRIORITY_DEFAULT"
 	MAIN "sercon"
-	STACK_MAIN "2048")
+	STACK_MAIN "2048"
+	COMPILE_FLAGS "-Os")
 
 add_custom_target(serdis)
 set_target_properties(serdis PROPERTIES
 	PRIORITY "SCHED_PRIORITY_DEFAULT"
 	MAIN "serdis"
-	STACK_MAIN "2048")
+	STACK_MAIN "2048"
+	COMPILE_FLAGS "-Os")