I'm a 21yr old 5'2 with 34dd breast. In your opinion am I too small for my chest size?

Hi. There is no 'normal' breast size or shape. I have seen patients with D cup breasts request breast reduction surgery because of problems playing sport. Breast reduction surgery improves symptoms such as back/neck pain and
allows people to be more active and it will be easier to find clothes that fit.
As with all elective surgery the decision in the end comes down to whether the
symptoms bother you sufficiently to have the surgery. Regards Dr Charles Cope

34 DD breasts have a volume of approximately 600 grams,
which corresponds to a diameter of approximately 13 centimeters. Two breasts occupy 26 centimeters and you add
3 more centimeters for the cleavage for a total of 29 centimeters width. You compare 29 centimeters with the width of
your chest in the anterior projection. However,
you have complaints of back pain and indents on your shoulders from your
bra. I recommend new techniques called
The Ultimate Breast LiftTM or The Mini Ultimate Breast LiftTM
to transfer the breast weight to the underlying muscle eliminating those
complaints. These techniques avoid the
ugly vertical scars of the traditional technique, maintain nipple sensation and
the ability to breast feed. This may be the
alternative you desire rather than the traditional reduction with poor esthetic
results.Best Wishes,Gary Horndeski, M.D.

Based on your description, you may be an excellent candidate for breast reduction/breast lifting surgery. Ultimately, you will receive the best advice during in-person consultations with board certified plastic surgeons.
Generally speaking, breast reduction surgery is one of the most patient pleasing operations we perform. By removing “excess” breast tissue, adipose tissue, and breast skin  this operation reduces and lifts the breasts to a higher position on the chest wall. By doing so, patients often find improvement in neck, back, and shoulder discomfort and find it easier to perform their activities of daily living and exercise.
Other than the presence of symptoms, the “ideal” patient for breast reduction surgery is one who is psycho socially/emotionally/financially stable, has an excellent social support system surrounding him/her, is capable of arranging enough recovery time, does not smoke and who has reached a long-term stable weight.
Patients considering breast reduction surgery should also consider the potential downsides (risks/complications) associated with the procedure as well. Poor scarring, for example may be associated with the procedure.
When the time is right, I would suggest starting with the American Society of Plastic Surgery and/or the Aesthetic Society of Plastic Surgery to obtain a list of well experienced board-certified plastic surgeons.
Then, I would suggest you visit a few surgeons whose practices concentrate on aesthetic surgery. Ask to see lots of examples of their work and preferably speak/see patients who have had similar procedures done. I hope this, and the attached link, help.