Remember, it's not uke's fault that he's big and tense. Being big is just genetics, and being tense is just because he's new. Think of it as an opportunity to learn to deal with that kind of energy.
That's a really good point.
Some of the bigger, stiffer guys have taught me the most about going around force, leading, and extension. The heavy weights also made me learn to appreciate the art of the facial (http://www.youtube.com/watch?v=0byR147UUjA
For the OP's pleasure: http://www.aiki.rs/shihan_5kyu_e.htm