We have a guy in class who is 6' 4", 250 lbs. I love/hate sparring with him because he's a monster (151 lbs and 1' 2" my superior) but if anything I do is working against him it will work against almost anyone.
One thing we are learning against such a big opponant is "cutting him down" so that vital areas are accessible. We do this mainly by attacking his knees.
Except, this is what happens when you attempt to knee someone in the soft part of the knee and end up hitting the hard part.
Of course you need to be very careful when attacking any knee while sparring because they are easily injured so this impact was pretty whimpy. Nevertheless, it left my knee looking a little worse for wear. He just stood there and laughed at me while I started making whining noises.
Moral of the story:
When you attempt to knee a knee, go deep.