I can't find any information about why the NUD blockchain is smaller than a normal blockchain. They have one sentence in the announcement thread claiming to have a smaller blockchain and then nothing to explain why it's smaller. It's extremely unlikely they've done anything to significantly lessen the size of the blockchain. I don't even think the NUD code is open source so you cannot check anything they claim.

EDIT: it is open source and based on a quick glance it definitely appears to use the original blockchain model.