Would it be reasonable to go from 38H down to a full C cup? I'm 5'3" and weigh 155lbs.

This size changes easily accomplished. I don't think any antecedent weight loss will have much effect on the operation.

At
size 38 each 200 grams corresponds to 1 cup size change.You would require 1000 grams removal from
each side to achieve a 38 C.This size
may be disproportionate for your body and may look inappropriately small.I recommend a new technique called The
Ultimate Breast Reduction™.The breast
tissue is reshaped creating upper pole fullness, elevated higher on the chest
wall and more medial to increase your cleavage.The weight of the breast is transferred to the underlying muscle
resulting in immediate pain relief without excessive reduction.This technique avoids the ugly vertical scars
of the traditional technique, maintains nipple sensation and the ability to
breast feed.Best Wishes,Gary Horndeski, M.D.

Thank you for your question. While going to a C cup is reasonable, if you lose a significant amount of weight after a breast reduction, your breast size will also go down. This may in fact take you down to a B cup or less with significant weight loss.  You may want to consider going to a D cup  and losing the rest with diet and exercise.  Once you have been reduced, the only way to go back up is with an augmentation so it may be a good idea to be a bit more conservative about the reduction.  You don't want to over-reduce the breasts. Best wishes.