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: 
A critical mass of U-235 fuel is only 17 centimeters in diameter and weighs 52 kilograms. A sphere of U-235 the size of the earth would be impossible to assemble because it would go critical and explode long before you could get it that big.

The heat generated at criticality is not what sustains the chain reaction, it is the neutron flux released by the fissioning uranium atoms.
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 .