Question: Imagine a standard $^{235}\text{U}$ pellets scaled to the scale of Earth, I wonder, would it explode or just meltdown? I believe despite overshooting the critical mass, the heat produced is still too small to start sustained chain reaction? Right?

Answer: 
It is worth noting that [there is no such thing as a critical mass](http://blog.nuclearsecrecy.com/2015/04/10/critical-mass/): there is a critical *condition* where there is enough mass, neutron reflection and other shape properties to make a chain reaction run to completion rather than produce some heat or a fizzle.

Still, a huge sphere of U235 will not just sit there. [50 kg of uranium-235](https://pdfs.semanticscholar.org/e182/291a3aa2754e02a3bcd8f8e1693539819ba1.pdf) is the *bare sphere critical mass* of uranium-235, that is, it will become critical even without reflectors (see the linked paper by Chyba and Milne for calculations). For a huge sphere the neutron mean free path $\lambda\_f = 16.9$ cm will be far less than the radius and all neutrons in the core hit something. That some of the surface is too "cold" to react does not matter if much of the bulk does.

There is a final issue: can the explosion overcome the gravitational binding energy of a huge lump of uranium? I we assume one earth mass of uranium at normal density I get $R=4210$ km and $E\_{binding}=3.4\cdot 10^{32}$ J. U235 fission releases about 82 TJ per kg. So the nuclear energy that can be released is $4.9\cdot 10^{38}$ J, or 1.4 million times the binding energy. So, yes, it will explode even if the reaction is fairly inefficient: even a fizzle would turn it into plasma.
It is conceivable that Jupiter might have a radioactive center the size of a planetoid or planet. The high specific gravity of uranium and thorium suggests they might co-locate towards the center of a planet. The possibility of going critical seems more than just an academic question.

While uranium and thorium compounds (like oxides) have a lower density, one might expect these compounds to be chemically reduced in planetary interiors. Perhaps the Earth might have experienced, or is currently experiencing, fission in its interior.