This name should be listed as Jewish rather than Hebrew it's clearly not a Hebrew diminutive form (coming from a Hebrew speaker who lives in Israel). [noted -ed]
This name is the Ashkenazi (European Jewish) version of Batyah or Batia. In modern and Sephardic (Spanish and middle eastern Jewish) Hebrew, the letter ending the word "bat" (daughter) is pronounced as a "T" - thus "bat". In Ashkenazi pronunciation, the letter is pronounced as an "S" - thus "bas" - for the same word, "daughter". The "yah" or "ia" portion is the first syllable of one of the Hebrew names of G-d.