its size, size, size...
divide your pixels by at least 200 to see how many inches it will print to sharply. (and by 300 would be better with modern printers)

The problem would be that you've made them in a pixel based program (or am I wrong) - which means you can't really scale upwards. So with all those lines you've got I'd propose to make the next maps in Illustrator or Inkscape - those are vector and can scale to the moon and back