The density/mass of the cubes may be different. I believe it was Archimedes who discovered that for an object to float, it must displace it's own mass worth of water. That is, a 1kg sphere would need to displace 1kg of water to float. If it can't do that, it sinks.
They are much much smaller and much much cooler despite being much much closer to the sun.the gas giant's mass allows for constant movement and friction which warms up it's core.
