A person could also not withstand none of the blows Ifrit or Batraal deals and yet none complains about that being unrealistic. In my opinion, the weapons look great, the size is great seeing that Ifrit is huge, his horns would be huge too, the weapons, like everything on this game is very detailed, they are not 'just' big weapons.
Please developers, continue with the great job you are doing, the weapons are amazing.
Edit: To everyone comparing the designs with other games I tell you this: One thing I don't like about some FINAL FANTASY fans is they think they are above other players in terms of taste, they feel they are refined gamers above all others and therefore inspiration from other franchises can not come into their beloved FF. I love the tip they might have taken from Monster Hunter (I feel this is where they go this, Yoshida being a fan of the games) when designing this weapons. I say keep at it and apply the FF spin to it. I am looking forward to new armour found in other FF's too.
