I tend to do that as well. Usually starting with a clarification of the expectations.
Then I ask what budget they are prepared to allocate to meet said expectations. If the answer is "none", I ask them which other expectation shall be lowered. This may seem confrontational, but it isn't really. If you want me to do more stuff without giving me the means and time to do it, something will suffer, and that needs to be made explicit by me, because I am the person facing the consequences when this something else can't be done adequately.
I was once asked to become the responsible electrical engineer for my institution. For them this was just a position they had to fill for legal reasons (otherwise they are liable in case of damages) and I have the qualifications, so they asked me. Then I explained to them that legally my role is only seen as valid if I am given the time and the means (equipment, room, powers to stop failings, etc) to do the job properly. Otherwise they would still be liable. I then asked them if they were prepared to dedicate that amount of my work time and an extra budget to that role. Surprise, they were not. So I declined. As of now I am still not sure where that liability went.
Too often management wants to have their cake and eat it too, and pointing that out isn't rude. It is one thing to ask someone who is idling have the time to take on tasks that are close to their job. But it is a totally different thing to ask someone who is already at 110% capacity and doing the job of three people to take on yet another job.
This is bad management. It is flattering that I am apparently good enough at my job to be constantly offered new responsibilities and asked advice at projects, but that is how you lose people like me.