Don't get too hung up on it. Everyone's body responds differently to pregnancy and those who eat normally throughout can't understand the overwhelming, faint-inducing need to eat that some do.
I put on 5st 
with DC1. And do you know what? I was fine, my baby was fine and I lost it all afterwards. My mum ballooned when pregnant, as did her sisters and my cousins, so I think it's a family thing (well, that's what I'll keep telling myself 
).
I didn't worry so much about the weight gain for DC2 and 3 - stayed away from the scales so I have no idea what I put on (fortunately I don't speak kilos, which is what they used at my antenatals), but it was a lot. Again, all was fine and it all went, although it took longer with DC3.
This time I have already put on a fair bit at 19 weeks and hope not to put on quite so much by the end, but that's because I've got lots of lovely clothes in dying to get back into (and stay in, as DC4 is definitely our last 
). But at the end of the day, I can't not eat when I'm hungry and even though I'm 39 I know it will eventually go when I put my mind to it.