comics - Is Superman always holding back?

This is a quote from the Justice League Unlimited cartoon:

That man won't quit as long as he can still draw a breath. None of my teammates will. Me? I've got a different problem. I feel like I live in a world made of cardboard, always taking constant care not to break something, to break someone. Never allowing myself to lose control even for a moment, or someone could die. But you can take it, can't you, big man? What we have here is a rare opportunity for me to cut loose and show you just how powerful I really am.

Has Superman always been holding back, even in the comics?

Answer

Is he always holding back? I don't think so. What would his reason be to hold back against Doomsday? To hold back is to assure his own defeat. Certainly against normal humans he has to be delicate, because he could easily kill them.

The real issue is that Superman has always been plagued by the inconsistencies of his powers. Sometimes he struggles to lift a large car or bus, other times he is able to move an entire planet.

http://en.wikipedia.org/wiki/Superman#Powers_and_abilities

As originally conceived and presented in his early stories, Superman's powers were relatively limited, consisting of superhuman strength that allowed him to lift a car over his head, run at amazing speeds and leap one-eighth of a mile, as well as an incredibly dense body structure that could be pierced by nothing less than an exploding artillery shell. Siegel and Shuster compared his strength and leaping abilities to an ant and a grasshopper. When making the cartoons, the Fleischer Brothers found it difficult to keep animating him leaping and requested to DC to change his ability to flying; this was an especially convenient concept for short films, which would have otherwise had to waste precious running time moving earthbound Clark Kent from place to place.

As the super villians became more clever and more powerful Superman had to be changed in order to always win. And thus the great powers race was started:

Writers gradually increased his powers to larger extents during the Silver Age, in which Superman could fly to other worlds and galaxies and even across universes with relative ease. He would often fly across the solar system to stop meteors from hitting the Earth, or sometimes just to clear his head.

This created a problem of course. Eventually Superman is so powerful there isn't really compelling story to be told:

Writers found it increasingly difficult to write Superman stories in which the character was believably challenged, so DC made a series of attempts to rein the character in. The most significant attempt, John Byrne's 1986 rewrite, established several hard limits on his abilities: he barely survives a nuclear blast, and his space flights are limited by how long he can hold his breath.

And yet the great powers race continues...

Superman's power levels have again increased since then, with Superman currently possessing enough strength to hurl mountains, withstand nuclear blasts with ease, fly into the sun unharmed, and survive in the vacuum of outer space without oxygen.

In the animated Superman series he starts off relatively unpowerful, and seems to be come more powerful over time. It is possible that like a muscle, as Superman uses his powers and stretches himself to his limits, his powers grow to accomidate these experiences.

http://en.wikipedia.org/wiki/Powers_and_abilities_of_Superman#After_John_Byrne

Though still largely invulnerable, Superman has been shown to be susceptible to damage from other yellow-sun powered Kryptonians such as Power Girl, Supergirl, General Zod, Non, and Ursa (as well as the Daxamite heroes Mon-El and Sodam Yat); and characters with powers comparable to those of Kryptonians—including the heroes Captain Marvel, Icon, Martian Manhunter, Orion, and Ultra Boy; and the villains Atlas, Bizarro, Black Adam, Cyborg Superman, Darkseid, Despero, Doomsday, Lobo, Monarch, Mongul, Ultraman, Validus, and the current version of Brainiac (when connected to his ship). Most notably, he has consistently been portrayed as weaker than Superboy-Prime, a character who possesses powers closer to those of the Silver Age version of Superboy.[22]

Superman would be a fool to hold back against the characters who are at the same power level as him.