Yes, but with the headline and the guy talking 'massive'... they could have included a 'relative to' or something? I was expecting some kind of deep sea leviathan.

'Dog-sized animal species discovered...' gets a lot less clicks.

The relation was made in the headline. It stands to reason that a massive half billion year old animal would be massive relative to other animals from a half billion years ago.

Does the average person have a good sense of how big animals were 500 million years ago vs. 250-300 million years ago? The latter of which was quite big, and any 'massive' from there on would be fitting by modern standards.

The average interested person would have, yes. That would include anybody who's watched any of the relevant PBS Nova programming, or any of David Attenborough's work, etc. - you don't need to have kept your head in the journals to be informed at that level.