It's normal (and unavoidable) for your nipples to swell in a flange tunnel to fill the available space.  No matter how large you size up, you'll probably still swell to fill the tunnel.  But swelling puts pressure on milk ducts making it harder for the milk for drain thoroughly, increasing the risk of plugged ducts and reduced milk supply.  That's a big reason why we find that most people drain most thoroughly and comfortably in the smallest size they can comfortably fit into.  

If you're finding that your nipples are swelling noticeably, it may mean you need a smaller flange size.  If your nipples stay swollen, try any (or all) of these treatments: 

  1. Arnica gel (safe for breastfeeding, available at Amazon here and in most drug stores in the wound healing section, wipe any residue off before pumping or nursing)

  2. Cold packs

  3. Cabbage leaves (let me know if you haven't heard of this and need info)