I had the exact same problem with my first work, so let me tell you how I dealt with the problem.
I was trying to aim for 60k, but my book ended at 20k. There were several reasons the book came up short. The main was that I had tried to write the book without any thinking- sort of like the pantsing or discovery approach (see this question). Like you, I did not stop at all.
I found that this approach did not work for me. While I hate plotting in detail, I found that I have to do some plotting. I usually write 20-30 bullet points scenes, which I then try to flesh out. Since I had done no plotting at all, my steam ran out, and the book finished at only 20k.
The other reason was, the book was mainly dialogue and action, with sparse description. Now I try to flesh out my work. The best way I found is to imagine you are in the scene you are writing, and then write the scene.
The third reason was- I had too few characters. If you only have 2-3 main characters, they can only have so many problems, and your book can only be so long.
Now , to answer your question, yes for some people, 1st drafts are much shorter than the final works. This advice flies against the accepted dogma/canon, that says you have to write a long 1st draft and then cut it down. For a few us (the silent suffering minority :) ) this advice is not true.
If I aim for 60K, my 1st draft comes out at just 40-50k. I add another 10-20K in my other drafts.
However, in your case, the gap is huge. I can tell you what I did with my 1st project. 1st, I added more characters, with their own problems. I also add more scenes, taking another project I was working on and merging it.
Do a rough calculation. If you write scenes that are 1200 words long, and you need a 60K book, that means you need about 50 scenes. You can be very flexible about how you use the scenes. If I plan 50 scenes, I usually throw more than half away, and add new scenes as I'm writing. However, I find that while the plot comes out by writing, I do need some scenes or bullet points to guide me towards the end, or else my muse gets bored and types "The End."
So my advice to you is: Put the book away, for at least a month, then come back to it. Read through it, and figure out where you can add more plot. Can a minor sub-plot be expanded? Can the minor characters be given more important roles? What else can make the hero's life difficult(eg, he gets kidnapped by aliens while trying to save his girlfriend from the Mafia)?
Write down 40-50 scenes, and then start expanding your book. While the task may seem daunting, my experience is it gets easier as you get along.