For my application I care only about updates to identities (say pgp key changed in persons profile) but not about transfer of ownership, such as transfer of a nickname . Would in this case the download requirement of a full node be much smaller than full Blockchain ?
You can actually have a full Blockstack node and delegate blockchain calls to a remote Bitcoin node. So you don’t need the whole blockchain. You only need to process and store Blockstack transactions.
Also, you should care about name transfers, because they mean that the user has re-keyed. The key that owns the user’s name is the key that delegates to other keys, like PGP keys.
Recall that Blockstack Core maintains a mapping between names, public keys / bitcoin addresses, and zone files, which include references to profile data, which has been signed by the owning public keys or keys that the owning public keys have delegated to. Thus if you want to validate profile information, you need to know the owning key of the user’s name / root identity record.