Weird science. The size of the hero impacts his weight but not his force. Therefore, getting bigger, his weight is more + has his regular force. When he is small, his weight is less but he still has his force.
He is a bullet, when he is small. He is a giant bullet, when he is a giant.
Hope it helps. Stay bless. Enjoy the Vine. PEACE (+)
Originally, Ant-Man's mass was sent to the Kosmos dimension by means of Pym particles, or if he needed mass, when he was Giant-Man it was pulled from there. That's why he didn't weigh 180 pounds when he was the size of an ant and why he weighed more when he got larger.
They don't try to give too much of a scientific explanation for it in the comics. In recent years it has been explained that there are 3 axes on which Pym particles work. Size, strength and durability and this explains why when he is Ant-Man he retains normal size strength.