An stl file is just vertices and faces (triangular, as I recall). It's just data. When Creo reads it in it's just an exact copy of all the data into a model, represented with surfaces, one for each facet. It's a pretty horrible type of file to use for any further modeling - no true surfaces, a simple cylinder is a huge number of facets, etc. There's no way to read it in that makes it smaller.
You could always have the creator of the file use a larger tolerance, which would reduce the size of the file by representing things in a "chunkier" manner, but it will look worse, etc.
Sorry if you have to have an STL for a follow-on process, I've only ever used them as output files from Creo to make rapid prototype parts and the like.
