My dad has at various points in his life been obese, and he's a GP. I imagine that he has told people they need to lose weight, and it's more because the insurance mandates that he has to tell them.
Also, you can't judge someone on their weight (as I'm sure all of us know). Maybe the doc is battling with weight problems that you don't know about, and he's been working on it his whole life (I know my dad's definitely in that category).
You should try and take them seriously because in the long run they're telling you what's best for you. Obviously if they say to you "you need to lose weight", without giving you any help or suggestions as to how to do so, that's pointless and useless, and they shouldn't be saying it. But they should be telling you to go to a dietitian for help. That's what my (not overweight) doc told me when I first got pregnant.
He didn't tell me to lose weight, but rather that I should go to a dietitian so that my weight gain could be monitored, to help reduce the danger of GD during my pregnancy.