Why does Hashirama Senju possess such huge and terrifying chakra? In the past three Naruto manga chapters (chapters 621~623), the story goes like this: child days of both Hashirama and Madara, and he (Hashirama) is just like every others in Naruto: a normal human being. But when Orochimaru revived all four Hokage in front of Sasuke, Hashirama raises his chakra level to threaten Tobirama, as it would crack all the walls nearby or maybe the entire basement (chapter 620). How could have Hashirama acquired this kind of power (chakra)?
When Naruto had gone to Mount Myoboku for his senjutsu training, Lord Fukasaku had mentioned 


  only those who already possess "extreme chakra levels" can utilise
  natural energy to invoke senjutsu


Source

Now we also know, Kakashi mentioned Naruto having large reserves of chakra (excluding the Nine tail's chakra). And this is the reason why Naruto could complete the Sage training and enter Sage mode, thereby giving him access to a larger amount of chakra.

The same goes for Jiraya, as he had completed the Sage mode training, and could enter the sage mode himself.

Thus we can assume, once in a while kids are born with higher reserves of chakra.

The same can be assumed in case of the first Hokage and Kisame, they could have been born with a large reserve of chakra.

Reason for assuming this is, First Hokage could enter the sage mode himself, and as for Kisame, we know the sword of his, Samehada would only attach himself to a ninja with good reserves of chakra. (Since the eight tails had more chakra and a more "delicious" one samehada switched allegiance and joined Bee).

In case of Naruto, Jiraya and First Hokage, their sage mode gave them access to a larger pool of chakra thus making them more powerful. On the other hand, Kisame could get access to a larger amount of chakra with the help of Samehada, that would "eat" the chakra of enemies and supply to Kisame.

Nothing much have been mentioned about the third Raikage, so I can't provide any proof to support my speculations.