My lady and I have an on-again, off-again ANR (adult nursing relationship) and have been trying for years to induce lactation, but she didn't even get milk when her son was born. There are a lot of ladies out there that induce or continue milk production after they wean the little ones, simply because it feels good, and some do it for the boobs.
My understanding of the process is that during puberty and pregnancy, the hormone shift causes the milk glands in the breasts to develop, of course the process is more dramatic in pregnancy, but the general process is related. As women age, if they're not putting those glands to use, they undergo 'involution' to fatty tissue instead of the denser, firmer glandular tissue. I honestly don't know why some women lose a significant amount of breast size after they stop lactating, that's not something I've researched, but it is common. (As a side note, I read somewhere that women taking estrogen-only birth control for many years tended to have gained some breast size as a result, but higher risk factors as wel, probably due to the BC being fake estrogen alien to the body. The naturally high estrogen environment of pregnancy causes the glands to develop, then the sudden surge of progesterone at birth 'turns on the faucet' so to speak.)
So long as you're keeping the glands active and 'awake', they don't tend to shrink back down. For some women, that can be as little as their mate suckling twice a day for a few minutes, or manual expression a few times a day.
I haven't seen a lot about it here, but at the (prohibited O), there are a few older threads that go in depth, and at least one or two of the girls have had some level of success inducing lactation (without getting pregnant) and saw some results on the order of a couple cup sizes. Most of the herbs that women use for breast enhancement are actually galactagogues, which means they affect milk production. Any growth is probably a result of the breasts attempting (for lack of a better word) to lactate, without actually doing so. Fenugreek and Goat's Rue are the two that I remember most clearly as the top herbs on the list.
By the way, you'd find MANY appreciative people here if you were to document your progress with photos. I'm imagining a sort of time-lapse as your breasts and belly grow through your pregnancy
I've already got the OK to do the same with my wife if she gets knocked up. :p
Hope that wasn't too much of an infodump for you!
Welcome again.