Images can matter more than page count
A few high-resolution scanned pages can outweigh dozens of text-heavy pages because each scanned page may effectively be a large image.
Fonts and vector graphics also take space
PDFs may embed font data and vector artwork so they render consistently elsewhere. If those are the main contributors, image compression alone may have little effect.
Some PDFs compress much more than others
Documents full of high-resolution photos or scans have more room for reduction. An already optimized, text-based PDF may barely change.
The target depends on the purpose
Meeting an email attachment limit is different from creating an archival copy. For submissions, check both the required size limit and readability.