measuring ahead?

I just had my 32 week appointment today and my midwife told me it seems like I was measuring ahead. I was just curious how they determine this because my visit was like literally less then 5 min lol. she heard baby's heart beat, felt my stomach for a min or two and said "it feels like your going to have a nice size baby" she then asked me how much my first weight (she was 6pound 15 oz) and she yeah he might be alil bigger which I am so surprised because with my first I gain about 30 pounds and ate aton. with this pregnancy I gain about 12 pounds. she then measured my belly and said it looked like I was measuring ahead. just wondering this is something accurate or how they determine It?