The complete sentence is the following:
" As the size of the nucleus increases, more NEUTRONS are needed to maintain the attractive force. "
In fact, as the size of the nucleus increases and the number of protons increases, the repulsive electrostatic forces between them increase as well (because the protons have the same electric charge, so they repel each other). In order to hold them together, it is necessary to have more neutrons in the nucleus. In fact, the strong nuclear force acts between protons and neutrons holding them together, like a "glue", and preventing the nucleus from breaking apart due to the repulsive electrostatic force of the protons.