Breasts are made of fat, glandular tissue, muscle, and skin. If any of these things change, your results will be affected.
If you are in the healthy BMI range, you do not need to lose weight in order to have surgery. However, if you lose weight after surgery your results will be affected. The amount of natural breast tissue you have will decrease, so your overall breast size will decrease as well. There may not be enough breast tissue to cover your implant, so you could develop visible rippling, causing the skin envelope to become loose and the natural breast to droop down.
These changes might not be reversible by putting the weight back on, as you have no way of controlling where the fat cells go (and typically fat cells go anywhere but your breasts).
Alternatively, your breasts may not change, but the rest of your body could change so that the breasts now appear disproportionately larger.
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