Hey, the nipple on my smaller breast is definitely a swelling hog, as well haha. Nipples or areolas in general tend to be puffy when you have small tubular breasts, so what I do is I put a soft ball inside the dome and then pump to kind of push the fat/swelling back into the breast, so the outer sides of the breast can grow. I am sick of the ball just falling around inside the dome sometimes and never really end up sitting where I want it to. Someone else on this forum uses toilet paper rolls as a nipple stopper, she cuts them down as much as she needs to and then puts something on top of it (where the hole is) so the nipple won't have a chance to expand any further from that point on. Yoghurt containers sound like a good idea, as well, depending on the size of course.
Glad to hear it is going so well for you and I will definitely let you know if I am having success with my method :-)
I like how heavy my small breast feels after pumping sometimes!