Is there a some of kind of rule affecting the pronunciation of "es" coming at the end of a word? In some words I hear "-es" as "ɪz" and in some others I hear it as a "əz".
I also noticed that people having a British accent prefer pronouncing with "ɪz" more.
Some examples: boxes (əz (with schwa)) houses (əz) misses (ɪz) fixes (ɪz)