diff --git a/ports/esp32/boards/sdkconfig.base b/ports/esp32/boards/sdkconfig.base
index c84dd606b2..91e68c7bf6 100644
--- a/ports/esp32/boards/sdkconfig.base
+++ b/ports/esp32/boards/sdkconfig.base
@@ -4,6 +4,12 @@
 CONFIG_IDF_TARGET="esp32"
 CONFIG_IDF_FIRMWARE_CHIP_ID=0x0000
 
+# Compiler options: use -Os to reduce size, but keep full assertions
+# (CONFIG_COMPILER_OPTIMIZATION_LEVEL_RELEASE is for IDF 4.0.2)
+CONFIG_COMPILER_OPTIMIZATION_LEVEL_RELEASE=y
+CONFIG_COMPILER_OPTIMIZATION_SIZE=y
+CONFIG_COMPILER_OPTIMIZATION_ASSERTIONS_ENABLE=y
+
 # Application manager
 CONFIG_APP_EXCLUDE_PROJECT_VER_VAR=y
 CONFIG_APP_EXCLUDE_PROJECT_NAME_VAR=y