Image size can remain large due to leftover dependency packets. In any case this starts to become a complicated matter, so let's do this a bit more thoroughly:
1) Can you get Carambola2 to a working state (boot successfully) by using our pre-built firmware (http://pkg.8devices.com/carambola2/v2.4 ... pgrade.bin)?
2) Once you have a working pre-built firmware running, can you upgrade to newer (or older) pre-built firmware versions by using sysupgrade (and NOT from the bootloader) command? Try doing this few times by alternating between http://pkg.8devices.com/carambola2/v2.4 ... pgrade.bin, http://pkg.8devices.com/carambola2/v2.5 ... pgrade.bin and http://pkg.8devices.com/carambola2/v2.6 ... pgrade.bin
3) If everything worked fine in step 2, that means that the hardware should be ok. Now try building and upgrading CLEAN (the default one - no ffmpeg, etc.) firmware using sysupgrade.
4) Finally build and upgrade (also with sysupgrade) to your custom firmware (with ffmpeg or other libraries).

Please do each step in sequence and don't skip any. Also post the commands that you are using and if anything in that step fails - provide output of each command.