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!
Follow Mothering