Are you taking into account that large size creatures have a -1 to attack?
Male Minotaur MonH6; CR 4;
Large Monstrous Humanoid;
HD 6d8+18; hp 51;
Init +3; Spd 30 ft/x4;
AC 17 (+3 dex, -1 size, +5 natural), touch 12, flat-footed 14;
Base Atk/Grapple +6/+14;
Full Atk +9/+4 Two-handed...