"%mem" trouble of Gaussian

Hello, CCLers.
 I like to ask the effect of "%mem=xxx" on Gaussian98W.
 I've met curios phenomenon with G98W_ver.A7. However I modified the
 PC's memory "%mem=128MB" to "%mem=512MB" on same
 there was very little calc time difference between them(57min and 64 min!)
 If anyone met same phenomenon, please let me know.
 All responses, I will appreciate. And I will summarize them.
  Sincerely yours,
       Telkuni Tsuru     telkuni[at]venus.dti.ne.jp