Same situation here, OP. I'm an only child and my mother is difficult. DH has brothers and his parents are nice people.
From looking around at my family, friends and acquaintances, I see there is a certain correlation between having only one child, and being generally a difficult person to live with. The parents of only children I know usually have anxiety, are perfectionist, some of them feel easily overwhelmed and are rather self-centered, and sometimes that makes them obnoxious... so it makes sense that they wouldn't have a large family.
Of course it's wrong of me to generalize from my non-statistically significant sample and of course not all parents of only children have the same personality!