262144 is the size of the filesystem in the new firmware. The existing one has only 131072.
That means it was built with --build-var BOARD=esp8266-2M and included the boards library.
mongoose-os-smart-light does not include it and it’s built with the default 4M layout.
To build it with 2M layout, either add boards in its mos.yml or use --build-var FLASH_SIZE=2097152.