I think the first thing to understand is if you're accepting these small tasks because you have to or because you want to. I found that I would often find every little problem interesting and relevant and would accept working on it.
I soon became overloaded, and I decided that I would have only a certain number of slots open for projects. For example, two projects from this department, two from that and one pet project (my choice).
I would say no to any new projects that didn't fit in a slot. This doesn't work as well when the source of the problem is that you must accept all work pushed on you. In that case, you probably need to talk to your manager.
Think hard about if the problem is your inability to say no to an interesting project.