There are several factors that affect first draft length. Some make it run long; some make it run short. It depends on the individual writer's quirks. It's what revision is for.
1. A common (but not universal) first draft issue is bloated, wordy prose. Even if every scene, every line of dialog, every bit of description is necessary to the story, some writers' wordcount can be cut 10%, 20%, or more by pure sentence-level tightening. Compare
Walter began to wish that he had asked Melissa whether she would go out to dinner with him. to
Walter wished he had asked Melissa to dinner.
2. Sometimes a first draft contains larger chunks that need cutting. Excessive description, long narrative explanations of backstory, even whole scenes that don't advance the story. Some writers put everything they think of into the first draft and decide later which parts are unnecessary.
3. Other writers get so wrapped up in getting the story down that they focus only on "what happened". They leave out description, their dialog is all talking heads, etc. In extreme cases, the first draft may be more of a detailed outline than a novel. Those writers have a lot of fleshing-out to do later. A very sparse first draft might gain wordcount by 30% or better in revision.
4. Too much telling and not enough showing can make wordcount run low, as well. Compare
Susan was angry. to
Susan stomped into the room, slamming the door behind her. She yanked open her dresser drawer and rooted furiously through it. She whirled around to glare at George. "What did you do with it? I know you took it."
5. (I'm sure there must be a 5. I just can't think what it is right now. 
)
