First of all, my breasts were only very rarely sore (except when poked right in the nipple) and I got perfectly fine development. Soreness is *one* symptom, but it doesn't correlate directly to how much/if they're growing. Secondly, breasts go through phases of growth - sometimes they're changing daily, it seems like, and sometimes they're "preparing" and nothing seems to happen until you have a growth spurt literally overnight. Thirdly, the one cup size smaller thing is a myth that gets perpetuated because it sounds plausible and nobody understands how bras work anyway; just like cis girls, you'll get what your genes say you should get from puberty. (It may not look quite as impressive on a trans woman with a bigger frame than she would have had if she'd had a cis puberty, alas.)
And lastly, it's been a week. Don't panic yet. 🙂 Cis girls spend 5-10 years growing breasts, and so will we.