Golden-doodle public access trained

Ok I got a golden doodle from my aunt who works with an orginazation that trains servie dogs. They train dogs to pull wheelchairs and such so the normal size of the golden doodle is perfect. I got my little girl because she is tiny. She still helps me but is to small to pull anything. I know they are a mixed breed ( I don't mean that in a bad way so please don't jump on me for it, some people won't like me saying that) and she was supposed to get the height of her mom the standard poodle and her dad the golden retreiver. I was just wondering what could have kept her so much smaller than her litter mates. She is only 20 inches high and 35 pounds or so. Please let me know what you think. Email me at specklove@hotmail.com I understand some people will think it is bad of me to put the email address on here but I have so many sites to rememeber I will probly forget about this one. Any ideas would be helpful. Thanks