If you have been around for a while you may remember back in the day that there was a time you could only put 140 characters in an SMS. Then someone devised a method to string them together (it's a bit like a blockchain, funny enough). Basically, in the database, this is how graphene is probably storing the posts, as strings of 256 bytes, normally large enough for any graph data.

This is not a sustainable method of storing large blocks of data, not least of all because it's going to walk all over the block size boundaries.

You may have bumped into a problem of a similar nature with the FAT32 filesystem, and a large video file over 4Gb in size. The filesystem cannot do this because it hasn't got enough space to store the map of where all the pieces go.