I actually train mostly women. I also have a client who is "too skinny" - in her words, and she wanted to gain weight but, "good weight". However her reasoning was mostly just because of her arms - her upper arm was way thinner than her elbow joint. That bothered her. It may not bother other women, though. She's a Nurse, too, so atleast has an idea of what's "healthy" and what's not. However, she was just chasing a "look".
Anyway, I don't know much about women. The more I think I know, the more clueless I become. So I just accept I don't know very much and try and just listen, which seems to help. Concentrate on the "look" you want and forget about numbers - that would be my advice. No one can tell you what look to go for. I have clients who think Jessica Beil is far too muscley, and clients who think she has the perfect body. It's individual.
Body Image is a huge issue - for men aswell Remember the majority of men train to "look good nekkid". However it's still very individual. Some guys think bodybuilders are "too big", some guys think you're small unless you're "heyoooge".
Anyway, my advice to Karen would be to keep doing what you're doing. To be honest BMI, despite being a national standard as an indicator of "health", actually tells us very little about how healthy we are. For example my BMI is 29. I'm bordeline obese. However my bodyfat is low. So, the "look" vs "the number" is wildly off. In November my BMI was 31, and i was at the bodyfat level of only-kinda-sorta-having-abs-in-the-morning. However I certainly didn't look obese, and deffinitly do not look like a BB. I get family members telling me I shouldn't get "any bigger". I get gym members/friends telling my I need to bulk up. However, I know what i'm training for, and I enjoy training for it. I generally make progress. Since I make progress and enjoy it, I feel good. Due to this, other peoples comments just fly over my head.
I guess the point is just enjoy the process of what you're doing. It'll "empower" you. People who don't train can subconsciously try and put you down. They generally don't know they're doing it, so don't take it personally. It's like a subconscious form of jealousy. You should take it as a compliment