Why are my breasts triangular looking? (Photo)

At one day post operative you should not be worrying about the shape of your breasts. You are very early in the healing process and your breasts will continue to change. Regards, 

With the size of the implants, the swelling that normally happens after this surgery, and the fact that you have tight tissue, your triangular look is totally normal and to be expected in the early post period. Your breast will change as the swelling resolves and your tissue stretches. Continue to followup with your surgeon and follow his;her advice and directions for care.

I can tell you that you will have a great result.  You are so early in your healing process.  You have a large breast implant that your chest is trying to accommodate.  You will see some significant improvements over the next several weeks but it will take at least 3-6 months for your implants to settle.Best Wishes,Nana Mizuguchi, MD

Hello and thank you for your question. Congratulations on undergoing
breast augmentation. It is normal to have worries and concerns
initially after any surgery as there is a lot of trust patients put in
their surgeons to deliver the results they desire and the moment of
truth is after surgery. Many
people envision their breasts being much larger after they first unveil
them, but once the initial
worries of having undergone surgery subside, most patients see that they
received what they expected. Considering you
are only 1 day out from surgery, it is very common for patients
to wish they went bigger during the initial post-operative period. My
advice to you is to hang in
there, as your breasts will look even better, but you need to be
patient as your implants settle. Right now they are squished by the
activation of your pectoralis major muscle and tissue swelling due to
the inflammatory response after surgery. Once they are in their final
position
and lower pole of your breasts is not as tight, your breasts will appear
even
larger and rounder; usually this process occurs as the pectoralis major
muscle relaxes and the implants are no longer squeezed upward and settle
into their ideal position. This process usually takes 1-3 months, but
can take up to 6 months.
Remember that each side may heal and drop and fluff at a different rate. Also, make sure to relay your concerns to your
plastic surgeon as most value the patient-doctor relationship and
want to be first in line to help their patients. If you are not happy
with
the size once they are fully settled, then you can consider pursuing a
revision. I hope this helps.Sincerely,Dr. Sean Kelishadi

Your own breasts superimposed over your "behind the muscle implants" are giving this composite effect.  It will evolve to a more natural shape and form over next several months. Be patient with it.