Sounds about right.
The installation size for Bungie’s upcoming ambitious MMO-FPS has been revealed to be 40 GB on the PlayStation 4, and it seems like it will be 40 GB. Considering the size of some other games on the PS4 and Xbox One (Titanfall, inFamous: Second Son, Wolfenstein: The New Order), that sounds about right, really.
Of course, with the sheer amount of updates and patches that a constantly persistently emergent growing world like Destiny’s will entail, it is entirely likely that the size of Destiny grows to something far beyond 40 GB soon enough- even if it were a standard game, actually, it would almost certainly get multiple patches and updates that would add to the size immensely.
Do note, though, that this is only the size for the PS4 version of the game- one has to wonder how much space the game will end up taking on the Xbox One, or even better still, the PS3 or Xbox 360.