His arms were just holding the weight, not pushing it. Now if he threw the island into Earth orbit he'd be stronger than Super Saiyan Goku. I'll admit he stronger than base Goku, but thats about it.
One you still don't understand how lifting works. If his arms aren't buckling under the weight he's still lifting it and resisting not only billions of tons of weight but the vast force needed to propel a man and a multi kilometer diameter island into space coming in the opposite direction.
Go up to a wall put your hands on it with your elbows slightly bent. Now push forward with your legs. Notice your arms are having to work to stop you going face first into the wall. You are having to provide a reaction equal to that provided by your legs. Superman is just using flight as propulsion rather than his legs.
Two your way over rating Goku's strength. How do you compare base Goku who struggled with 40 tons to someone who can launch an island into space. SS2 Goku struggled to push a building sized mountain in half.
Assuming a 4km wide cube made of diamond ( close enough to Kryptonian crystal) It would have a mass 54 billion tons. 4km is a good bet as the average depth of the Atlantic ocean is 3.2km
Using a handy online calculator http://www.1728.com/gravity.htm. A 54 trillion ton object would have a Gravitational force of 2.1611e+17 to overcome to move it at a 100km distance from Earth. It would take exponentially more energy to move it at any great speed. I could work out how much more but eh.
That energy is the equivalent to lifting 1 e+13 or 10 billion tons. Yeah that's comparable to having 10 tons on each limb. This is also bearing in mind he had almost no strength left at this point.
Picccolo "I...didn't even...see it! It looked like there was a flash of light...and..."
Doesn't a gun do the same thing? flash of light, bullet moves to fast to see.