compression level using miniz having no effect

Question

I'm evaluating using the low-level routines in miniz and testing different compression levels with differently sized inputs. Level 0 always gets a little bit larger (as expected) but the other levels all result in the exact same size for the same input. They are getting progressively slower (and level 0 works) so setting a different level has some effect. I can decompress everything back again.

   tdefl_compressor compressor;
   auto flags = tdefl_create_comp_flags_from_zip_params(level, -15, MZ_DEFAULT_STRATEGY);

   auto status = tdefl_init(&compressor, write, &target, flags);
   assert(status == TDEFL_STATUS_OKAY);
   tdefl_compress_buffer(&compressor, data.data, data.size, TDEFL_FINISH);