For some reason I feel the need to weigh into this debate... oh wait - it's about breasts growing, that's why!
I think that there is a logical conclusion wherein everyone is right and Linsey is perfectly natural.
Consider:
1: In her more recent work, Linsey is certainly not as big as she was just before her miscarriage. Certainly I think it's true to say that if you review the "Huge Changes" set she's by far the biggest she ever was at any stage. Afterwards she was smaller, although I agree she did not seem to go down to her original size.
So, we can say that she got bigger whilst pregnant and didn't keep every last inch afterwards.
2: From my own observation of my friends, many of whom I've known from teenage years, there's nothing that says that 16 or 18 is some kind of magical cut off point when a persons body shape stops developing. Indeed it seems relatively common to put on a cup size between 18 and 22 or so without any particular weight gain.
So perhaps we could say that there's no real reason to suppose that Linsey had actually finished growing up when she started modelling.
3: Finally, as commented by someone in the "Natural models who've grown" thread, the one (virutally) guaranteed way to get bigger boobs is to put on weight.
Now, consider that we know different people tend (from hereitary tendancy or whatever) to deposit fat in different parts of their bodies. We have the poor "pear shapes", for example, where every ounce hits the hips and nothing goes elsewhere.
Given that, generally speaking, the business part of the breast (as in the milk generating bits) are a similar size whatever the cup size, we know that the rest is more or less fat. If you look at someone the shape of Linsey, Lizzie (above), Ewa or, for an extreme example, Merilyn you can see that the breasts are certainly the place where they tend to store fat. Disproportionately so in fact, which is why "naturally slim and stacked" is so terribly unusual in real life.
So we can see if Linsey or Merilyn put on a pound or two, the majority is going to land on their tits, with smaller amounts going elsewhere. To the extent that either of these girls could almost certainly put on several pounds in weight and add a cupsize without the added weight being obvious elsewhere.
Take, for a slightly more obvious example, Kerry Marie. If you look back at her first shoots she's really not that big up top (I'm speaking relatively of course). She gets consistently bigger throughout her career (something like a small F then to an advertised HH now (although I think she's doing herself down from her latest shoots)). Thing is though, it's not until a couple of years in that it really becomes obvious that she's steadily gaining weight. Of course it's obvious now because other parts of her are now noticably large (and none the worse for that) but it's still her boobs that shifted the most. You can also see a more subtle effect on Liz who's already been seen in this thread. In her later shoots in which her boobs are larger, there's the gentlest curve on her stomach that wasn't there before and there's one less rib visible.
So in the course of her career we've got to get Linsey from maybe an F to an HH (or whatever Score billed her as). As far as my understanding of bra sizing goes we've maybe 3 or 4 cupsizes to go (depending on how many double letters we go via). Perhaps if she'd not really finished growing when she started modelling we have 1 - 2 sizes. Say she gained a few pounds over the years and we have another two. Add in the possibility that she put on a little weight whilst pregnant and didn't quite lose it all afterwards and we have the perception of not losing her pregnancy size.
Anyway, feel free to bash at my logic. Personally, having thought that lot through, I'm now surprised she's not bigger!