The big fillet radius prevents load-induced bending from cracking the journal off from the cheek. All cranks (rods, etc.) are subject to fail where the cross-section changes radically - this is one reason why 4340 is used more for rods than cranks - it doesn't tolerate as much change as 4130. Even a change from .020" to .060" radius is huge.
There's even a technique for increasing the radius without reducing the journal. The new radius is undercut in the side of the adjacent cheek, tangential to the journal surface but ending normal to the cheek.

But... the crank isn't stronger overall - it's just less likely to break at that spot.