My breasts are heavy after a growth spurt, should I wait till I'm sure they've stopped growing before I get a breast reduction?

Breast development will vary from one individual to the next. Development begins with the onset of puberty. This process may continue until a young woman reaches her mid 20's. For some it will begin early and others late. Some will have significant growth while others very little, both of which can be a source of distress.
With regards to your specific question, it is not uncommon to have breast growth which continues until your mid 20's (the average in the literature is 24 years). You are not alone.
It is important to remember that the breast is both gland and fat. Breast development is obviously hormonally sensitive, however, breast size will also be sensitive to weight gain/loss. Thus your non-surgical options are limited to weight reduction (or discontinuation of medical hormone therapy/birth control).
Once stabilized, I would encourage you to seek out consultation with a board certified plastic surgeon (ASPS). In my experience, reduction patients are among the most satisfied, reporting a life changing experience (for the better).

Your case is quite impressive and you may have a condition known as virginal breast hypertrophy, (VBH). Rapid breast growth in this situation is due to hypersensitivity to estrogen and while it usually happens before or around puberty, it can occur up to the age of 24. While breast reduction surgery would probably be your best choice, there are other options that may be better for you including medication which can block the estrogen effect on the breast tissue. Please select a plastic surgeon board certified by the American Board of Plastic Surgery and preferably a member of the ASAPS, (American Society of Aesthetic Plastic Surgeons).