OpenWrt only able to use 50% of the ROM

Dear community,

My device is CR6608 running Openwrt 22.03.1. The ROM of this device is 128MB and 256MB RAM. I noticed that I am only able to use 64MB (58.6MB) for installing the software in /overlay partition. In the storage section, there are two partition: Disk and temp space. Is there a way to make use of all 128 MB instead of 64MB or this how the system is designed? Please advice.

root@G:~# df -h
Filesystem                Size      Used Available Use% Mounted on
/dev/root                 4.0M      4.0M         0 100% /rom
tmpfs                   122.4M   1004.0K    121.4M   1% /tmp
/dev/ubi0_2              58.6M      4.8M     50.7M   9% /overlay
overlayfs:/overlay       58.6M      4.8M     50.7M   9% /
tmpfs                   512.0K         0    512.0K   0% /dev

Tmp is half of your RAM.

What does cat /proc/mtd say about the storage?

Indeed, it uses RAM is temporary storage. My ROM has 128MB, it is expected that there should be around 100 MB usable.

root@G:~# cat /proc/mtd
dev:    size   erasesize  name
mtd0: 00080000 00020000 "Bootloader"
mtd1: 00040000 00020000 "Nvram"
mtd2: 00040000 00020000 "Bdata"
mtd3: 00080000 00020000 "Factory"
mtd4: 00040000 00020000 "crash"
mtd5: 00040000 00020000 "crash_log"
mtd6: 00400000 00020000 "kernel"
mtd7: 07980000 00020000 "ubi"