I suspect that there is not a lot you can do to force the bamboo to produce larger culms until it's ready. Bamboo size up on their own schedule, and it already sounds like they are doing well. Thinning won't help, and watering in winter won't make much of a difference. If you want to give them fertilizer, give them something like 0-10-10 in the fall, which will encourage rhizome growth, but not unnecessary leaf growth. In the spring, a general fertilizer or a slow release fertilizer will be fine.

Thanks for the input. From my research on this thread I think you are right--the bamboo will do what bamboo does.

Use lawn fertilizer, that is a type high in N in spring and fall. Be careful with the P, once you put on too much you have to dig the dirt out to get rid of it - it leaches extremely slowly.

Re pruning: ok anytime except when the plants are shooting.
Prune out older culms - rule of thumb is take out 1/3 of grove every year after it has matured. Lots of controversy over the effect of pruning. My observation is that taking out the older culms seems to have a positive effect the next year.
In my madake grove, I lay down steer fertilizer in late fall. I don't water in winter - just let nature do the job. Horse manure is ok as well plus many horse owners are eager to give it away. I've been using a product called "Earth Juice" type catalyst on my smaller to medium bamboo with good results. You mix it with water so it goes a long way. It builds great root systems.
I'd also suggest adding mulch to your grove every year. Rgds