Re: The Problem with Being a Big Guy
As a fellow "big guy," I know how you feel. I'm 6'4" and 250 lbs., though I'm doing my best to reduce the weight some. A couple of points that might be relevant:
A former sensei reminded me that 50% of the time that we are doing aikido, we are doing ukemi. For me, that means that the "aikido part" does not only apply when I'm throwing someone. It should also be present when I'm uke. I see my goals as uke as (1) to create a situation where the nage can practice and learn the technique; and (2) an opportunity to do my own practice.
I try to be open and sensitive to nage, to let them have the full experience of executing the technique. They take my balance, they do the throw. I try not to just collapse to the mat without provocation, and I try to not resist too much so that nage is stymied. I try to go where they lead me, by being sensitive to their movement. I don't always succeed, but it's my goal. I think being flexible is more martially sound than being stiff or strong, or locking down someone.
Now, there is absolutely an exception to this: sometimes nage wants more resistance. In that case I give more, or even a lot, but only when requested.
By saying "more martially sound," I mean this: I think locking down a nage with strength is in general a little risky. As you have noticed, it doesn't affect someone who is experienced. Besides that, it is often the case that if you are strong in one direction, you will often be weak in another. Or, worse, the nage might actually be stronger than you, in which case you might get injured. If the nage is not so experienced, it can be frustrating and in some cases counter to learning.
So, as uke, what does locking someone down get you? Best case scenario, by using more strength you get tired--more tired than you need to be. Middle case, your nage gets frustrated. Worst case, you get injured. Of course, these are not the only possible outcomes, but I think using a lot of strength as uke is sub-optimal.
I think I disagree with your friend who said that by being loose, you open yourself up to being cranked on harder. If you combine flexibility with sensitivity, awareness and a healthy sense of self protection, I think you are in a more martially sound position than if you are bearing down on someone with all your strength.
This is how I understand it, of course, and I don't always succeed at all this, but it's one of the ways I practice aikido as uke.