or it could be because bitcoin's memory pool (aka transaction list) gets larger, whereas namecoin stays the same.
not really
namecoin has the same growing rate like bitcoin. u should know that the protocol regulates the speed... thats just an unqualified comment
Bitcoin has way more transactions that namecoin. That has nothing to do with the "growing rate" you refer to, which I interpret to be the 10-minute time inverval between blocks. This increased transaction rate could expose a theroretical leak in bitcoin but that doesn't mean that it isn't present in namecoin too.