Re: Larger/stronger opponent
Kick this guy out. Imagine how devastating he will be if he sticks to aikido!! (just kidding)
I haven't seen a couple of comments:
1. Humans have a bell curve of size. Aikido works well within the bell curve, but as you move farther outside the curve into the extreme ranges, you have to make necessary adjustments to interact. Just as this large guy has special handling, so do our smaller friends (for example, adjusting nikyo to grab fingers instead of a wrist).
2. It is not this guy's fault that he is big. We recoil from large partners and sometimes [incorrectly] assume the guy is meaty, oxen, and stiff. Oh, and purposefully obstinent.
There is already some good advice out there so I won't dwell on re-iterating posts related to the above obersations, I make this comment - disguish with your partner when you are learning and when you are training. Leanring = figure out how to do it; training = practice it to work. During learning, discourage resistance and emphasize cooperation. During training, train - if your uke is being difficult, change. Also, there is nothing wrong with re-inforcing to your partner that you can competently deal with resistence (especially if you have prior martial experience or senority in aikido).
Eventually, giagantor will either get the message that muscle isn't everything, or he won't. You have a unique ability to assist this guy to not only learn aikido, but learn more about his body's strengths and weaknesses.
This is one of those things that as the original post mentioned, we don't like the reality that size matters. Size does matter, we just have a larger bell curve in which we can operate. It also directly addresses our fallibility because its tough to admit failure when a particularly difficult uke thwarts our aikido. Grrrr. It must be them. Grrr.