Grandslam and Tallboy were completely different beasts.
Those were high explosive earth penetrators.
Both this new device and Daisy Cutter are designed for airburst and lack fragmentation effect due to the different skin material.
As to size (this from CNN):
MOAB: 21000lbs, 18000 of which is tritonal explosive
BLU-82 (Daisy Cutter): 15000lbs, 12600 of which is an ammonium nitrate component (fertiliser).
MOAB both has more explosives than the entire weight of the BLU-82, and that explosive is more powerful too.
From the photos, it looks like the MOAB has some gliding capability enabling standoff release which would help deployment against defended targets (BLU-82 has to be dropped more or less directly overhead).