I've been reading up on how estrogen will eventually reduce the muscle size of a trans woman undergoing HRT.  Does that mean that the female hormone regiment will cause the bladder decrease in overall size?

It's not the estrogen but instead the lack of testosterone. As for the bladder, after 34 years I can still carry about the same amount of water as I  could 34 years ago as measured both times in the hospital. It possible to have issues if you are not careful and get infections and some people inherit bladder issues but they don't seem to be related to HRT.

Thanks Dena for sharing, but I'm curious what you mean 'inherent bladder issues'  with regards to my own condition I predominately produce calcium oxalate stones.  Most of them I have successfully passed normally although the last one I needed to have lithotripsy to shatter the stone so I could pass it. 

Some people have difficulty holding large quantities of water and have to visit the rest room often. I don't know if their bladder is smaller or if it's more sensitive but it can happen. Women also sometimes have leakage while men sometime have issues with swollen prostates that make it difficult to go. Eliminating testosterone tends to eliminate most prostate problems. For something so simple in design, it seems like a lot of things can go wrong with the plumbing.