Low tire pressure in 39.50/15/20r?

i just bought a 97 doge ram it has a 14" lift with 39.50 super swamper irok on it i was checking the pressure in my wife's tires and they are normal size i figured my tires being so much bigger would have more pressure but i checked them and they only have 15psi that's not a typo ether but they are full i cant put anymore air in them i was wondering what would cause this i explored a lot of things they are not filled with nitrogen and do not have tubes in them dose anyone know what would cause this thanks