5'1, 120lbs petite, will 400cc HP be too big/top heavy for me?

Dear Washingtonian,   Front and side view pictures would have been very helpful. The correct Implants size selection depends on your chest and breasts circumference measurements in inches  and your desire.      In my practice, I take the measurements myself, because, many women over estimate their breasts size , since Victoria secretes make the cup size smaller, so women feel better about their breasts size.   You commented that in clothing you look like a full B cup size, so , going from 260 cc implants to 400 cc implants will make you 34 Full C cup size. To become 34 D cup size you will need implants with volume of 475 cc.  34 D cup size is a relatively big size, but when done correctly it can appear very attractive and natural. It is critical that you get a nice cleavage and for that I would recommend to place the implants through a sub areola incision, which allows for nice dissection of the cleavage area under direct vision and feel. Sub mammary crease  incision is too far for obtaining consistent nice cleavage.    Always, consult with experience board certified plastic surgeons who operate in accredited surgery center for your safety. Most importantly, check the before and after in the photo gallery, to make sure that they are numerous, consistent and attractive with nice cleavage, perky, symmetrical and natural looking.                        Best of luck,                                             Dr Widder

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 cup” or "top heavy" 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, and the attached link (dedicated to revisionary breast augmentation surgery concerns), helps. Best wishes.

It's impossible to say for  sure. It sounds  excessive for your frame, especially if you  want a totally natural look. Your PS will  may use sizers at the time of surgery and he/she could make a better determination of the proper volume to achieve the desired D cup. The PS  should have a variety of implant sizes  available.