That's true, but there is a surprising amount of soft tissue there as well, and they will get thicker over time.
Very, very slowly. Big wrists are a thing you're born with or get by growing up doing manual work. City boys like me have small wrists.
If you do lots of grip and wrist training over a long period of time the tendons in your wrist may get thicker which could make your wrist slightly larger in circumference.
Weight training might affect them slightly as, afterall, bones are a living tissue and adapt to stress. However, the enlargement via weight training is pretty significant, atleast in my personal experience. Your wrists could be put to shame by someone who has never touched a weight in their life. They do get thnkcer as you get older, though. So there is hope for both of us. :icon_chee