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.
Interesting. So majority of people isn't informed enough to see this problem at all and those who know aren't voicing it.