BIrling - "A heavy-looking rather portentous man in his middle fifties"
As he's depicted by "quote" this allows him to be a microcosmic representation of the older generation
"heavy-looking" this is Priestley showing the audience that Mr Birling is wealthy without explicitly saying it. In 1912 having a plumper frame would reflect on your wealth.
His heaviness could be deeply metaphorical as it is a physical assertion of the solidity of his status whilst also standing to represent his undisciplined tendencies and showing lack of control.