That it is certainly more egregious, it's more morally weighty to take a person's life than it is to steal a pencil. In fact, if this wasn't the case you could never solve an ethical dilemma where you have two particular things that come in conflict with each other. When taken in isolation both would be wrong. Like whether you ought to obey the government or obey God in preaching the Gospel. We can do that because we know that some things are more morally important than others.
Common sense seems to dictate that, too.
But when someone says sin is sin, there is another aspect they may be getting at. That is, any sin disqualifies you from heaven and brings you under God's judgment. Even the least of the sins. That is true. James says if you are guilty of one sin and not of another, you are still guilty of breaking the law, so you are guilty of the whole law.