Reading this quote this morning makes me think it's time to re-read MERE CHRISTIANITY again...
"The sins of the flesh are bad, but they are the least bad of all sins. All the worst pleasures are purely spiritual: the pleasure of putting other people in the wrong, of bossing and patronising and spoiling sport, and back-biting; the pleasures of power, of hatred. For there are two things inside me, competing with the human self which I must try to become. They are the Animal self, and the Diabolical self. The Diabolical self is the worse of the two. That is why a cold, self-righteous prig who goes regularly to church may be far nearer to hell than a prostitute. But, of course, it is better to be neither."
I don't think Lewis is espousing "small" sin at all here, but instead he is talking about two different inclinations toward "sinning," similar to what Jesus mentioned in the Sermon on the Mount when he stated that what goes into a man doesn't make him unclean, but that which comes out of him does, and the idea that to call your brother "fool" is on equal footing with the act of murder itself. One is a natural inclination of the natural body we must resist, and the other is a deliberate choice after weighing the differences and still choosing to do evil. One is an act of the spiritual will, one than can hide behind a self-righteous act by avoiding the animal sins of drinking, sleeping around, etc. but living nonetheless in the spiritual ones that are easily dressed up and hidden (the pursuit of power, greed, etc.). A prostitute (to use Lewis' word) may be honestly in a bad situation and want out or may be in the only situation he or she has ever known (even acknowledging the wrongness of the lifestyle), but a deacon who hides behind church power to feed his greed and abuse of others (for example by "disallowing" mixed race couples to feel comfortable in church or to make sure that what he wants to happen for the direction of the church body because he gives the most money in tithes and offerings) has willfully embraced the diabolical self and is spiritually choosing the sins of the spirit. Does that make sense?
In another way of saying it, the sins of the animal self are sins of impulse while the sins of the diabolical self are sins of methodical, willful evil choices.
Sins of impulse are equally "bad" but can be more easily overcome in time while sins of willfully embracing evil traits and character require more work and faith to overcome. And sadly, the church today tends to focus on the sins of the natural self, best known in the adage of "no make-up, no dancing, no sex" rather than the admonition to live like Christ and have the mind of Christ in all you do, especially that which goes on beneath the surface, that which people may never see.
And sometimes because it doesn't reflect poorly on our churches or because we're blind to it, our churches today often empower and back those who regularly indulge in the sins of the diabolical self. It's far easier to spot a drug user with a filthy mouth than a well-dressed and "proper" church goer who runs his company and/or family like a selfish, greedy, power-hungry tyrant.