The sad answer is that the creators of either thing consistently demonstrated a lack a sufficient understanding of basic physics or even logic to actually gauge their strength/power. For starters, the energy needed to blow up the Earth is about 4 x 10^33 joules. The dbz characters readily apparently summon that sort of energy from their ostensibly < 100 kg bodies, and withstand that sort of energy when hit by it, yet simply training under a mere 100 to 1000 earth gravities is taxing on their bodies. Then Superman has to strain to catch an airplane and stop it from crashing, but in the last Christopher Reeve movie, he moves the moon to block the sun from nuclear man. There is no consistency across the board in either one.
A rule of thumb, don't take these kind of things too seriously. After all, both Superman and DBZ is a work of fiction. Trying to find logic in these things is futile. Besides, trying to look at everything logical and scientifically takes the fun out of things. Well, as Kamina says, "kick reason to the curb and do the impossible". That's what makes fiction more fun then real life. It's not bound to things like the laws physics, and can almost do what every it wants...depending on the imagination of the author.