Will 640cc be too big for me? I’m 5’4, 145lbs. (photos)

Hi and thank you for your photos and questions.  While choosing the right sized implant for each individual is a detailed and specifically tailored process, just from looking at your preop photos, I do not think your own breast skin and breast tissues will support such a large implant.  They may look good initially but eventually will likely bottom out because your tissues will not be able to support it.  You can see from your preop pictures that your implants are already weighing down your breasts.  A revision mastopexy with perhaps a slightly different/slightly larger implant should give you a lovely and more importantly durable result.  Best of luck!

More upper pole fullness can be achieved using several different strategies including larger/wider implants, a more cohesive silicone (gummy bear-type) gel in a silicone implant, or overfilling a saline implant.  Whether the 640 cc implant your surgeon recommended for you would be appropriate would depend on your measurements and the size of your current implants.  Yes, a 640 cc implant is a large implant, but your wish photo is also consistent with a large implant, so that recommendation may achieve your goals.  A word of caution, however, is that large breast implants cause strain on the natural breast tissues and may thin your skin and settle more over time than a smaller implant.  Your wish may be achieved in the short-term and cause problems in the long-term.

Thank you for the question and pictures.   You will find that online consultants will not be able to provide you with specific advice in regards to selection of specific breast implant size/profile. Ultimately, careful communication of your goals will be critical.
Generally speaking, the best online advice I can give to ladies who are considering revisionary breast augmentation surgery (regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully.  Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals  with your plastic surgeon. This communication will be critical in determining  breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as  “natural” or "D or DD cup” or "too big" etc means different things to different people and therefore prove unhelpful.
 Also, as you know, cup size varies depending on   who makes the bra; therefore, discussing desired cup  size may also be inaccurate.   
3.  Once you feel you have communicated your goals clearly,  allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals.  Again, in my practice, this decision is usually made during surgery ( after the use of temporary intraoperative sizers).  Viewing the patient's chest wall in the upright and supine positions, with temporary sizes in place, help select  the best breast implant size/profile for the specific patient.
I hope this helps. Best wishes.