Not as big as I want to be (Motiva implants). If a year later, everything has dropped, can I get another surgery and go bigger?

There are two separate yet related questions you have posed. The first relates to the surgical plan and a proper pre-operative informed consent with your surgeon. The second relates to your breast appearance and your level of satisfaction. A precise answer to your question regarding upsizing cannot be provided without a physical examination in person, or at least photographs. That said, if your breasts would accommodate a larger implant, revision surgery can certainly be considered. If it was your understanding that you would be receiving 380 cc implants and you received 280 cc implants, it is possible that you did not accurately hear your surgeon's recommendation. If there was a true change in the surgical plan, it is best to discuss your concerns with your surgeon.If you remain convinced that your level of satisfaction is indeed dependent on being made larger, this is something you should also discuss with your surgeon. If you feel less confident with your surgeon you can seek a second opinion. In general, from a surgical standpoint it is best to wait at least three months before considering revision surgery, but from a psychological standpoint I usually recommend waiting 6-12 months since, over time you may find that you have gotten used to you new appearance and breast size and you might find yourself happier with a more conservative augmentation. Best of luck to you. 

Of course you can consider a different cup size in one year time. Just make sure you will consult with a European Or UK Board plastic surgeon who can quantify exactly how much your tissues (breast, skin etc) can stretch without any damage.