The SteamPeaks account does not own ArtBook, so no manifest of it has been read and there is no size to measure. The Depots section shows the sizes Steam reports for each depot; the history starts the day a free licence or a key adds the game to the account.
No silent builds: every build of ArtBook on an unprotected branch in the last 60 days had a patch note or an update event within a day of it, is under 24 hours old, or has not been judged yet. The hourly check fills this in.
The SteamPeaks account does not own ArtBook, so none of its files are in the index and nothing can be compared until a licence adds it. 2,396 games are indexed today.