What is an idiomatic way of saying 'Straitness is in the heart'?
Consider a Danish proverb brought to the US by early immigrants, Where there's room in the heart, there's room in the house (or home). Google
From Nebraska stem such proverbs and sayings as "Where there's room in the heart, there's room in the house," "Don't holler before you're hurt," and "As safe as a cow in the stockyards."
Encyclopedia of the Great Plains
A: Please, come in.
B: No, thank you. It's all right. It looks too crowded in there, I don't want to be more strain.
A: Where there's room in the heart, there's room in the house. Come on in.