It is way too soon to be sure if your girls are pregnant. The weight gain is most likely just because they are young and growing still. It could just be because you are feeding them better food than they had been getting. Or even just tastier food and they've been pigging out. But some of them may be pregnant. You'll have to wait and see.
I wouldn't say it is IMPOSSIBLE that the little 16g girl is pregnant, but pretty unlikely. It depends on whether her small size is due to her stage of development, or just because she is genetically programmed to be small.
I can often tell when my females are pregnant when they still have about 1 1/2 weeks to go, but only because I am familiar with the individual female and how they usually look, and they still sometimes fool me (or possibly reabsorb?). By the time they are 5-7 days from giving birth, it is much more of a sure thing, and by the time they are about 3 days from giving birth, it is pretty much impossible to miss with most females.
I would definitely separate the pregnant female you have in with your feeders, but don't be too surprised if she eats her whole litter. Changing her living situation so close to her due date may be too much stress for her. If she does raise the babies, then I'd hold back some females, because a mom that is that easy going is the kind of genetics you want in your colony.
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