Hi there. The Noogleberry domes should not be painful, and using any of the several padding methods should solve any discomfort. I'm not familiar with Bosom Beauty domes, but I do currently use the NB XL domes and began with the large domes.
What you probably have heard is that, when someone has switched from the large domes to the XLs, they notice more roundness than "outward" swelling and growth at first. This does not mean there is not any growth occurring, it's just that it is more spread out because of the nature of the larger dome width. Personally, I noticed this when I switched over, but it seemed to sort itself out in less than two weeks. This same situation can (and I assume does) happen with folks who go up any dome size, not just the XLs... your breasts are no longer being smushed forward into a too-small cup, and so with the wider cup, the swelling and such is more distributed across each breast.
If your current size is large enough that the XL cups is what you would need to start with, you probably don't need to worry about any of this though. Good luck with whatever you choose!
and are they almost the same size i mean the height wise?