I didn't think weight gain or how much you ate had any bearing on it.
I gained a TON of water weight with DD2. She was 6lbs., 3oz.
Ditto with DD1 and she was 7lbs., 1oz.
But saying that, I never gained much 'real' weight adn I could never overeat because they both either made me very sick or gave me terrible heartburn.
DS1 is proving no exception to this - I usually just eat two small flapjacks + fruit or a sachet of porridge with honey and dried fruit in the morning, nothing until lunch and then it's reheated lefotvers with yoghurt laced with fruit mixed in for pud and a cooked tea of some sort with maybe a few biscuits or a cup of fruit smoothie.
That's about it.
The baby is sitting very high up and I don't have a very long torso, so I constantly feel full.
I've never been able to manage large portions, but especially during pregnancy.