I flashed my WRT1900ACSv2 with LEDE and started configuring and installing packages. I soon noticed that the /overlay partition was running out of space even though the box has128MB flash (one of the reasons I went for the 1900ACS). Looking at System -> Software it turned out that only 23.5MB are made available to /overlay. Is there any way to make /overlay utilize more of the available flash memory? I know that I can also change to an extroot configuration. However, 128MB are more than sufficient and so I would prefer sticking to a non-extroot configuration.
Since there are two partitions (primary and alternate), the remaining firmware flash allowance (80MiB) is divided equally between both partitions, and then trimmed down further by the kernel image. So I think that's why you are left with that much space at the end.