After flashing fresh OpenATV 5.1 on DM800SE, there is only about 1MB of free space in flash. One of the reasons is some rarely used utilities installed by default on a box of such small flash memory, like for example nmap. nmap is an administrator's network utility, not an STB utility.

But the most critical reason from having so small flash memory left is some leftovers from open-oe in /var/lib/opkg/lists.
root@dm800se:/var/lib/opkg/lists# ls -l
-rw-r--r-- 1 1001 1001 0 Aug 8 13:32 oe
-rw-r--r-- 1 root root 29700 Sep 27 12:10 oe-3rdparty
-rw-r--r-- 1 1001 1001 349126 Sep 27 12:10 oe-all
-rw-r--r-- 1 root root 1947528 Sep 27 12:10 oe-dm800se
-rw-r--r-- 1 root root 0 Sep 27 12:10 oe-dm800se_3rdparty
-rw-r--r-- 1 root root 6454632 Sep 27 12:10 oe-mips32el

these files are not needed, since we don't use any oe-* feeds. They are leftovers of about 8,5MB. After deleting them we gain about 4MB flash space which is really important.

Same files are on other boxes too, but since they have much larger flash, they got unnoticed. VU+ solo2 for example:

root@vusolo2:/var/lib/opkg/lists# ls -l
-rw-r--r-- 1 1001 1001 0 Aug 7 14:59 oe
-rw-r--r-- 1 root root 29261 Sep 20 10:22 oe-3rdparty
-rw-r--r-- 1 root root 352252 Sep 20 10:22 oe-all
-rw-r--r-- 1 root root 6533151 Sep 20 10:22 oe-mips32el
-rw-r--r-- 1 root root 1064797 Sep 20 10:22 oe-vusolo2
-rw-r--r-- 1 root root 268 Sep 20 10:22 oe-vusolo2_3rdparty

You should change the building process in order to delete those files from the final image.