If you run into her at another BBQ or etc. this summer, you could always tell her, "You know, I didn't have chance to tell you when we talked last time - so much of baby weight gain is genetic; I've a couple good friends who've had small babies AND large babies"
(after all there are several in this thread
-- And share some of our stories.
Ina was a skinny thing, tall and skinny actually (so she looked even more skinny - 95% for height, 3-10% for weight). I was actually afraid someone would call us in to CPS! We were certain we'd have all skinny babes, after Ina -- after all, dh and I were our mothers' smallest babies too. Instead, we have a MOOSE in SJ. She's huge! Still 95% for height, but also for weight too. My mom had 7 babes, all about the same weight at birth (other than the preemie) - she had three CHUBS (#1, #3, #5), two skinnier ones (#2, #7), and two "average" ones.
In terms of the "bf so she's smaller" comment -- I know I've read in several places that bf infants tend to weigh about 1 pound less (as a cohort) than ff'd infants when they reach 1 year of age -- I think Kellymom talks about this; that bf'd infants gain faster than ff'd from 0-3 months; at the same rate from 4-6 months; then "lean out" more from 6-12 months. So, there may have been a partial truth to what she was saying/telling herself.
I'd imagine she's been getting a lot of pressure, and could use some support. If you get the opportunity to befriend her more, go for it.
Even though I knew we were doing everything "right," I honestly still felt like maybe I was part of Ina's weight gain issues -- until we did the very same "right" things with SJ, and had a big baby instead.
I too think it's bizarre - we want our babies to be TUBS (no one wants a 50/50 baby, they all want a 90/90) -- then start worrying about their weight when they're older. Incidentally, my mom's biggest baby is the skinniest of all my sisters, she probably weighs 120 pounds today after three kids!