Okay - I can speak to this. I'm no longer a size 18 - but I was, when I bought my first B-bag (so yeah - a b-bag makes you thinner!!!!) I'm a size 14-16 now though - WAY bigger than most of the girls on this forum. I'm five foot five.
My DH thinks the First looks BEST on me. I have to say I like it too, although I'm still insecure. First of all, he thinks the bright colors that I have them in - and the style - is the best of all Balenciaga, and so, as an accessory, they add the most to my look.
A big bag doesn't make us look smaller anyway - and a small bag can be just the right eye candy that pulls the eye in the middle of an outfit. You can delicately hold the first by its handles in front of you or on your wrist - and I think it totally makes someone my size look great.
The City, while it holds more (and I carry mine daily) doesn't have quite the punch as a bright First - unless you get it in a bright color (you'll want one eventually).
But, I'm considering collecting more Firsts - because of all the motorcycle styles, it's the one that really contains the original Balenciaga charm to the Nth degree and, well, I personally can't imagine someone my size carrying, say, an Eggplant Work (we'd both just be...big).
I am getting a Magenta City - I've got enough confidence in myself now, stylewise, that I think it will work for me. I'm too shy to post pictures yet - but, well - you should really think about how elegant a small bag looks in the hands of nearly everyone.
Anything smaller than a First would be lost on me, btw. But the first really works well for us bigger gals (I think).