I suppose it is to show that yes, your uterus is growing and therefore so must the baby. It can be affected by so many things though, you're right, but some Dr's still find it a handy tool to know that things are going OK. My dr has always used it and even when I have been 'ahead' he finds no cause for alarm in that, kwim? I know other Ob's who even do scans at every visit too to check the baby, so it is six of one and half dozen of the other as to what method the Dr will use to check that everything is growing.

And no, there isn't anything you can do to control how big you get and eating less isn't a good idea though. You could simply have a lot of liquor in there and there isn't anything you can do to change how much of that there is.