As a life-long athelete, and former steriod user, I can tell you that Bale's weight gain (and almost all H'wood body transformations similar to his) are done with the use of either HGH or anabolics. No two ways about it.
I am not being accusatory, as I don't have a problem with the use of chemical assistance at all, most people believe the lies told to you by your 8th grade health teacher that nothing good comes from their use, but that is a different thread for another day...
Remember, anabolics are legal, just not for muscle building.
I always laugh when I hear people talk about how easily a H'Wood doctor will prescribe multiple prescriptions for "stress" to someone like Anna N. Smith, but they absolutely don't think that an actor would go to the same doctor for anabolics. I't laughable, and you have your heads in the sand if you don't think it doesn't happen just about all the time.
Hollywood is chemically driven, whether its alcohol, illegal drugs, legal drugs used illegally, or any combination thereof.
Again, I have NO PROBLEM with Bale or anyone else using chemical assistance to change his body for a role, but if you want to debate it, I'm perfectly willing, as long as it stays a debate, and not name-calling.