I just went away for a weekend by myself (yay!) and while I was gone, I pumped about 25 ounces of milk each day. Does that suggest that my 17 month old is really drinking 25 ounces of milk each day? No wonder he doesn't eat all that much lunch! Do you think it's a problem that he is clearly nursing instead of eating (at least sometimes -- other times, he can't get enough food).
I think it's common at that age to nurse a lot! DD is 17 months old and sometimes it seems like she is getting mostly breastmilk - some days she eats LOTS of solids on top of it. I would guess she is nursing at least 25 oz a day. I could be totally wrong but I think most parents who are feeding cow's milk instead of nursing after one year feed about 3 cups of cow's milk a day, right? That is about 24 oz.