Well....It's completely possible but please do not get upset with yourself if it doesn't happen.
I have two sets of twins. The first set I was absolutely neurotic about eating all the protein I could, drinking lots of water etc, etc, etc. My babies were born at 38w6d, weighing 6lbs and 5lbs4oz.
The second pregnancy, I could not even stomach the sight of meat. Seriously, I would throw up. So, my protein intake was much less. Oh, I still ate healthy, drank water, took it easy when I could. But, I was not neurotic about it -- OH, not saying you or anyone else is, but just that I was. Bottom line is I listened to my body. Babies were born at 37w3d weighing 6lbs6oz and 5lbs5oz.
So, almost identical weights. 2nd pregnancy the combined weight was 6oz more.
So, I guess my point is is that as long as your being healthy about your choices, not overdoing things, listening to your body and being smart your body is going to do what it's going to do. Yes, by all means, make good choices and that will make it easier on your body to grow big babies, but please don't set a goal that under 7 lbs means you didn't do the right things. Make sense?
Take care and good luck,