Masts most definately care the weight of the boat they are attached to.

When designing a mast the most important input is the righting moment and yes if you step your mast on a dock and sheet it on in conditions that would normally make you fly a hull when on trapeze you will almost certainly break it. The only time this is not the case is where the builder as run into a minimum practical thickness limit and as just has had to build a stronger than desired mast.