The 'Candy' singer and his wife Ayda Field are due to welcome their first child, a daughter, into the world next month.
In an interview with Italian Vogue magazine, he said: "I got fat. Isn't it incredible? She has to eat for two, but I've gained her weight.
"It must be the empathy. I read about it and it says that sometimes husbands suffer same effects during the pregnancy as their wife."
When Williams was asked if he'd experienced morning sickness and backache, he replied: "Yes, yes, everything. If she was sick, I was. If she had bad mood, me too. This happened during the first three months."
Despite him taking pity on his pregnant wife, the former Take That star revealed that she is not as nice to him now as she was when they began their relationship.
He added: "[In the beginning she would say] how stylish I was, she always congratulated me for how I dressed.
"Now she criticises me. She asks me why I am wearing that sweater instead of the other one. The first year everything is perfect, forgiven and wonderful, but the pain comes after."