I vary in size, but the last time around I was and H, ( yes an H) right after baby was born. Usually with in 3 months of the birth I am a D or Double D.
I have used a sling for all 3 born children and intend to do the same for the one we are expecting now. I don't know how to tell you to make it work exactly for you since I cannot see how you are wearing your baby and how you are postioning and so on. I will tell you things that worked for me and maybe that will help.
1. I did not use commercial slings, they just did not fit well and were mostly impossible to nurse in ( for me) I sewed or had some one sew (depending on which sling we are talking about I have them in several colors) the sling from Elizabeth Lee design. They fit so well and are very comfortable. I had them all made with an extra 6" on the tail as well, this made for extra covering in some cases and made them easier for me to adjust.
One of the issues with commerical slings is that most of them have all the fabric in the tail ( the part you pull to adjust) sewn all together so that you have to adjust the the top of the sling ( the part baby's head can stick out of) and the bottom ( the part that holds and tucks under baby's little bottom) at the same time and both have to be the same. With the Elizabeth Lee design slings you can adjust one part much tighter than the other if you want, I found this to be invaluable, both in adjusting for comfortable nursing as well and to make baby safe and comfortable.
I also recommend that the sling not be made out of too stiff a material as that seems to make it harder to nurse comforably with it too. Try differing positions until it is working for you, I never even thought of breast size being an issue with nursing in the sling, but I had the home made very easy to adjust slings. I also would get good @ nursing that baby before you tried to put them in the sling.
Hope some of this helps!