the personification of capitalism
Mr Birling is depicted as "a heavy-looking, rather portentous man in his middle fifties"
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. The reason is that in 1912 having a plumper frame would reflect on your wealth.
His heaviness could be deeply metaphorical as it is a physical assertion of solidity of his status whilst also standing to represent his undisciplined tendencies and showing lack of control.