Isn't the bomb in 'Pacific Rim' a bit small?

In Pacific Rim their plan is to throw a 1.2 megatons nuclear bomb into the breach.

Although this is reasonably sizeable for a nuke (Hiroshima was only 12.5 kilotons, about a thousand times less powerful) I would have thought that they would use something like the 58-Megaton Tsar-bomba.

Is there something I did not consider, or is there a mistake in the movie?

Answer

They couldn't build a bigger bomb because they didn't have the material.

In an earlier version of the script we learn that they'd tried (repeatedly) blowing up the Kaiju and the rift with nuclear bombs and had since depleted the world's supply of weaponisable Uranium.

LETHBRIDGE : It doesn't matter. There can't be a bomb. We're past peak uranium...

JIAO : He's right. The mines have all but dried up. We have no single source that could provide enough material.

[Raleigh speaks up from the back. In an earlier version of the script we learn that they'd tried (repeatedly) blowing up the Kaiju and the rift with nuclear bombs and had since depleted the world's supply of weaponisable Uranium.]

RALEIGH : Yes we do. The Mark-1's. That's what we did with the last of the uranium. We used it to power the first Jaegers. Like Gipsy Danger..

On top of that, the film transcript makes it quite clear that the aim is to "collapse" the rift rather than simply blowing the enemy into smithereens. Their modelling may well indicate an optimum yield size rather than a mentality of simply hitting it with "the biggest bomb ever built" in the hope that brute force will work.

"And this is what we call "The Throat," the passage between the Breach and us. We know that it's atomic in nature.

I predict that the increased traffic will force the Breach to stabilize and remain open long enough to get the device through and collapse its structure."