I think that Act III is about equal in length to Act I (including the entire Underdark, Grymforge, and monastery/creche), but it clearly had a large amount of cut content and probably should have been 50 hours rather than 30. However, this would have resulted in BG3 being about 100 hours in length, which returns to an earlier point that BG3 really should have been two separate games, or rather a game and its sequel, each about 50 hours in length. Granted, it would have been bizarre to release a game named Baldur's Gate where the player never actually reached the city of Baldur's Gate itself, and only somewhat less bizarre for the sequel to have occurred almost entirely within Baldur's Gate. But this would have addressed not only the issue of the cut content in Act III but also the decline in how polished the game is after Act I, as well as permitting a higher level limit in the second game than exists in the actual BG3 Act III.