Perhaps it was because he was lighter, or due to a huge amount of luck. Being in the position to he decides to attack the large man. He makes make a cut from his left towards the right with as much force as he can master in the moment

(8d6)[1][1][4][5][2][3][3][2](21)

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And here we go! So, am using a long blades weapon aka the bastard sword. Due to having Long Blades 3 I add 3 bonus dice. Five plus three is eight. Now if your Combat Defence is higher than my roll I don't hit. But if I roll higher than your Combat Defence, I hit!