128Mb flash size but no enough space

I have an Sercomm H-500 https://openwrt.org/toh/sercomm/h500-s device that has 128MB of FLASH memory.
I have installed an openwrt image. Last 22.03.4
The image size is 5.5MB. (downloaded)
The overlay size is 33.1MB.

Where are the remaining 89MB?

Filesystem                Size      Used Available Use% Mounted on
/dev/root                 3.0M      3.0M         0 100% /rom
tmpfs                    59.1M    236.0K     58.9M   0% /tmp
/dev/ubi0_1              33.1M    164.0K     31.3M   1% /overlay
overlayfs:/overlay       33.1M    164.0K     31.3M   1% /
tmpfs                   512.0K         0    512.0K   0% /dev

Is there a way to increase this memory in an easy way?



Sounds about right, ~94 MB / 2 (dual-firmware), - (kernel+rootfs, ~6 MB), - overhead (ECC/ OOB).


Is there a way to use the image part? what is it necessary for? is this safe?