He could be exaggerating, either to glorify the legion, or to scare their opponents. The legion, while fearsome fighters, are given to bravado.
Crni Vuk also makes the good point of what type of combat we're talking about. 16 legionaries could quite easily pick off 64 NCR soldiers over the course of a few weeks by ambushing small patrols, attacking lightly-escorted caravans, and sniping. If those same 64 soldiers were holding high ground behind even rudimentary fortifications, with a clear field of fire, it wouldn't even be close. The legionaries would be cut down before they got within half a kilometre of the NCR. In an open battle, the better discipline of the legionaries would probably give them an advantage against poorly-trained troopers who would tend to panic, but we're still probably not talking a 4:1 advantage.