Well think about this, if you do remain small that means your baby isn't gaining weight (which is where some of the increase is coming from) and there is a lack of nutrients and adequate fluids in the uterus (which is where the rest of the weight is coming from) and in no time you'll either have a miscarriage, premature birth and possibly a baby born with defects, or worse for the both of you if you start dieting to remain small.
Quit worrying about weight! If you eat the recommended amount of food for you and baby and continue a moderate (not insane) exercise plan such as walking or swimming, you will not be overweight after the baby is born. Weight gain in pregnant women who eat and exercise in moderation is solely due to the baby growing and the increase in ambiotic fluid and it goes away almost immediately if not within a week's time after delivery.
There are rare instances of unusual weight gain during pregnancy due to complications including pre-ecclampsia and the more rare HELLP Syndrome for instance. Hopefully your doctor is testing and monitoring for that. If your doctor isn't telling you your weight gain is unusual, you're probably fine and within range. You can also bring it up that you think your weight gain is unusual, just in case you are experiencing early symptoms of complications.
Until then, do not do what so many fearing weight gain do, and go on a radical diet while pregnant. It isn't healthy or safe for either you or the baby.
Prior to both of my pregnancies I was 125 pounds and gained 20 to 25 pounds with each by the end of my term. Within a week or two my size returned to pre-baby status....and I didn't exercise or diet during pregnancy or after delivery.
I did BF, which is scientifically proven to help mothers regain their prepregnancy weight and figure. (You can read about this at www.llli.org)
Maybe that had something to do with it. I BFed my kids until they were a year and a half.