Previously I could stick to my plot and write fairly short stories. Now I find my I have to stay 'in character' and to satisfy my characters motivations I am forced to write longer stories to get them to the same end. Admittedly, the stories feel as though they have more depth.

Is there a correlation between the ability to write a novel and a writer's ability to characterise? Obviously people who are weak at writing characters can still complete a novel, but it seems a that a complex character requires a longer story. So people who write character's first and formost are more LIKELY to see a novel as thier natural medium.

Long, rambling, possibly inane answer: Characterization is more difficult in a short story because you don't have as much time or opportunity--ideally, your story will be tightly focused. I don't believe in putting in characterization for the sake of characterization--all characterization should be there for the sake of story. In a novel, you have some leeway to explore your characters.

And the skill of characterization isn't linked to the skill in creating, or desire to generate, deep characters.