My lips aren't as big as I had expected after Juvederm Volbella. Should I opt for half a syringe more? (photos)

Volbella is a subtle, soft, gentle filler. The amount of filler depends on many things and I would recommend that you see your doctor again and review your concerns.

Please give yourself another week or two to let the swelling subside after your treatment before deciding whether you are happy with your lips or not. With the hyaluronic-acid fillers, its best to start conservatively, as you did, because it's actually quite easy to add more until you get what you want. Please consult a board-certified dermatologist who specializes in lip augmentation if you do decide to add more.

I suggest you wait for two weeks after your treatment, then return to your injector and get more if you're still unhappy with your results. 

Only you know how you want your lips to look.  If it has only been one and a half days then you probably still have some swelling which will go down making your lips appear smaller.  My suggestion would be to contact your doctor and discuss your concerns with him/her.

You may need more fillers. The administration of fillers is a highly technical procedure. There are so many factors to consider to determine the best filler to use, the right amount, etc. Please contact a qualified provider or a board-certified physician with extensive experience in these procedures.

The amount of volbella needed for the lips is variable, although I find about 90% of my patients are satisfied with 1 syringe. I would wait at least 2 weeks before deciding on more fillers, as bruising/swelling can sometimes mask the final result. While some people are bothered by the swelling in the first 48hrs, others actually like the look and wish the lips would stay that size (they do not).