Create dropdown menu based on values from a List, via multiselect fields

I have a large dropdown menu, that is used for a series of multiselect fields. The results of these then create a list. Is there a way that will create a smaller dropdown for a series of multiselect fields that only contains the values from this list?

Currently the end user is able to use the longer dropdown list but as this contains things not previously selected, there is the chance of selecting values not selected earlier. Hence the smaller dropdown would be preferable?