I didn't say it 'melts it' - it 'destroys it' - it's common where resistors are under specified like you're suggesting, the soldered joint, the PCB, and the wire of the component are all ruined making repairs difficult.
I don't know what happens chemically, but the copper of the board, and the wire itself become impossible to solder to, and the old solder becomes crystalline.
Just do the job properly, apply sensible design considerations, and build a circuit that's made to last.