If the one I found the link to is what you're working on I agree that you should be able to work some rounds to make the neckline smaller after the rest is done. I'm not sure you'll like the result. It could significantly alter the style. I would consider starting with the smallest size to make the neckline smaller and then enlarge for the bust. It could be done with increases to actually make the garment larger all around and then decrease them back out or it could be done with short rows to add fullness in the front where it's needed and the back would then have the same fit as intended in the original pattern. For someone "skinny as a rail" it's easy to make the rest of the garment too loose.
In both knitting and sewing I've coped with this problem for many, many years. What both crafts have in common IME is that trying to make the neckline smaller is much more difficult than trying to make the bust larger.
Just my thoughts FWIW. YMMV and I expect it will. I think it's just as cute as can be and hope it turns out to your liking.
I should never overestimate my basic arithmetic skills.