Generally if you are already overweight, they don't want you to put on too much, as you don't need to put on the extra fat stores that some women do need.
If I was her I would ask to be referred to a dietician, I put on nearly 20kgs with my first pg, and it was weight i really didn't need to gain. With my second pg I saw a dietician and only put on 7kgs total and lost it all plus some when DS was born. It does sound like the ob needs to learn some tact though!