Almost same situation, me and my wife were shopping, and she said: look over there green dress.
When I took a look, I saw a woman on her early 40's, 5'8 brown mid long hair, a little bit thick but her boobs were big, like G or so, yeah, big, but not huge.
"Those are some serious boobs" said my wife, here in my country the C cuppers are the big ones, but then behind this thick mom, were 2 young ladies comming out from a store the first one in her late 20's it seems to be the sister of the lady in green, 5'5 short Black hair, average body, walking by her side was her housband wich was carrying their newborn, which explains the interesting part a pair of breasts which I estimate on the H cup league, wobbling on a stretchy gray tank top, with a little muffin top at the clevage, but behind, of her, (and here is the sight of the year) someone that has to be the daughter of the lady in green, this girl was somewhere between 17 and 20 no more, She was 5'6, black straight hair, and skinny like 80 maybe 90 pounds, jeans, an unzipped hoddie, and a white "f*ck Barbie" t shirt, this girl was the one wich shocked us, her breasts were big, not like her mom's or her auntie's, DD's maybe even E cups, but on a girl of her frame, those boobs looked like L cups at least, the "F*uck Barbie" logo was deformed by the curvature of those boobs, my hands were full of shopping bags, and as I was with my wife, I couldn't take any photos or vids of the trio... Walking to our car my wife said, "poor girl, if she gain weight or get pregnant, those boobs will ruin her life" and start the conversation about, food, hormones on food, food industry and blah blah blah.
But yeah, in other sense of the same question, that skinny E cupper, gaining weight or even better gaining weight during a pregnancy, would make her boobs L cups easily.
