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 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.
Yes , it explodes ...
From many points at the same time , because it contains a very great number of elementary critical mass if highly enriched 235U fuel : it is only a big 235U bomb .

Under 20% enrichement ( approx. value ) , there is not critical mass for 235U , so nothing happens .