Storage Module Limitation

Hi I'm trying to put a website on ESP8266 4MB Flash, but then when i try to save files into Storage module, I found that I can't write any file size larger than 12k

require("Storage").write("test","a",0,12500);

will return

Uncaught Error: Unable to find or create file
 at line 1 col 44

Isn't there a ~4MB Flash storage for me to store stuffs?

p.s. here's how I initialize my ESP12

python3 esptool.py --port /dev/cu.usbserial-1410 --baud 115200 \
write_flash --flash_freq 80m --flash_mode qio --flash_size 4MB \
0x0000 "boot_v1.6.bin" 0x1000 espruino_esp8266_user1.bin \
0x3FC000 esp_init_data_default.bin 0x3FE000 blank.bin