Metformin , 500 . 1000, or 2000??

i have been taking metformin since 3 weeeks now my first week i had 500 mg , second week 1000 mg, but my dr. didnt tell me to take 1500 , or 2000 mg a day ?? any idea why, and am asking bcz some of teh ladys were talking about higher doses of that mediicne, so am wondering y ?? and also , am very very skinny , am 120 pounds and my height is 5.6 inch ( do u think thats y my dr. gave me the highest to take 1000 mg ) , i have pcos, but am not overweight .. i think am loosing weight since i started taking my medicine bcz am having less appettie and not feeling like having any food , wat should ido i reallllly dont wanna loose weight coz am gna look like a mummy , !