What expands faster, a liquid or solid?

It is a combination of the specific heat of water (coke is mostly water) and the high volume and mass of liquid in comparison to the mass of metal. You might have 500 grams of liquid that is expanding and only 20 grams of metal. So the metal might actually expand more, but since the volume and mass of the liquid are so much higher then all that liquid continues absorbing heat and expanding each and every gram of that liquid leading to more total expansion than the expansion of the little bit of metal.

You also have to factor in the expansion of the CO2 in the can.CO2 is a liquid when first mixed in the can. But it's critical temperature of CO2 is only around 31C which means anything above 88F and the liquid can't stay and turns into a gas. The pressure quickly builds the higher the temperature after that. So in the sun, the temperature of the CO2 could exceed 110F and the pressure of the CO2 alone could exert 10psi on the can walls, even with no expansion of the water or metal. So the CO2 could split the can with enough heat because of how rapidly it builds pressure as a gas

EDIT  : I read SOC's answer after I posted this and I concur. It is most definitely the CO2 being the biggest factor for the explosion because the gas doing the phase transformation (from liquid to gas) expands dramatically compared to what a liquid or solid would in the same phase