I'm almost 5'4" (ughh so close!) and I was 93lbs when I got pregnant.
I'm 102 now with a BMI just under 18 (still considered underweight even though this is how my body is)
I think overweight women are more susceptible to other health issues that put you at a higher risk for PE and HELLP (i.e. vit D deficiency, hypertension, PCOS (although I still think underweight women get that, too), high cholesterol...you name it)
I also think some people with just extremely fast metabolisms can go under the radar with certain diet related health issues...especially when they aren't eating correctly and not exercising.