Why does low resolution PDF output result in file sizes the same as high resolution PDF output?

Our publishing engine outputs huge file sizes, even when the low resolution job option is selected. Because of this, we end up using Acrobat to reduce the pdf file size. This led us to question whether the publishing engine is compressing graphics when outputting to pdf files using the low resolution job settings (which are set correctly to compress). Does anyone have some insight or recommendations on other avenues to investigate?  We are using Arbortext 6.0 M140 with eps graphic format.