I'm disappointed with my breast augmentation. I'm a week post op and I'm tiny already! Is this likely to change?

It is common to have some concerns after one week.  Things will still change with time.  If you were uneven before, and you had identical implants placed on both sides, then you will likely remain uneven.  Some of this may improve.  They may not get bigger, but the shape may change as things settle and swelling disappears.Discuss your concerns with your surgeon.  I usually ask my patients to wait 6 months before considering a revision because swelling and gravity will still change things. Often at the end of that time, the things they worried about are no longer an issue.  

I'm sorry to hear about the concerns you have soon after your procedure.
You should be aware that a significant percentage of patients at your stage and recovery will feel that they are too big or (more commonly) too small.
I would suggest you wait at least 3 additional months before you evaluate the end result of the procedure performed. This waiting time will allow you to (potentially) physically and psychologically adapt to the new body image. Many patients who have your concerns soon after surgery, end up very pleased with the long-term outcome of the procedure performed. In other words, patience and time may be your “best friends”. 
Best wishes for an outcome that you will be pleased with.