Actually this depends on  glyphs design. If glyphs are simple (have no many nodes and contours) there is no much potential in reducing the file size. But if glyph images are the result of scanning and tracing and have many details (like on the image below) they can be optimized. There are Clean Up and Simplify commands in the Contour menu for that.

The second good possibility to reduce the size of the font file is to get rid of hinting. You need to customize the export profile so that FontLab would export without autohinting:

The last tip is to use composite glyphs where possible. When Aacute, Agrave, Acaron etc. consist of A, acute, grave and caron they take less bytes in the final font even in the OpenType PS format. Extending glyph repertoire using composites is faster and more effective.