I've found that it is harder to potty train the little ones.  I had one that was only 3lbs. and I never did get her potty trained.  I had to sell her due to her size and me having small kids.  It wasn't safe for her.  (She was suppose to be a 6-7lb one.  Her parents were 7 and 5 lbs.)  She was about 20 mos. old when I sold her.  I still see her and she still isn't 100 percent potty trained.  She is 3 now. 
