It's absolutely enormous. Was it revealed why it's used?
This is a Minergate block:
http://explore.moneroworld.com/tx/bba343e5e172f07f623fb34c0cbd26519567cc374c5d61d468d370714f3d5b66
This is a regular pool block: http://explore.moneroworld.com/tx/7cddf2da09ec9c890d14ad18b9df7d4cd88e96103a739760a06dc5f91541ba05
Notice in the Minergate block there is this huge tx extra thing: 01c4230d59c83b3042fd82999cfda5be6b77c1673d9442d1f21c7ab9215bdc6c18021198414900000000000000000000000000000321004bff195014248ae22838853bcf293b8945f9baaac693c2d05bafe2f0f3e6c69e