I wouldn't worry at all! Horses grow at all different rates, and actually, being on the smaller side at this age can be an advantage, bigger breeds and horses that grow/mature fast are at a bigger risk of joint injury or things like OCD in the hocks.
My QH was kind of a peanut at a year old, but he ended up right about where he should be (mom was 14.2, dad was 15.1, he is about 14.3 so perfectly between them).
Here is him at 1 year and 1 month old
This is him at 1 year 3 months, you can see he is a bit butt high, they tend to grow in their butt, then their hind ends catch up, kind of like that over and over again
and him at 1 year, 11 months old, if you can imagine him under the fluff, he tends to turn into a wooly mamouth in the winter if he isn't heavily blanketed
Hope that helps! Sorry to have no comparisons with mom, after he was weaned she moved to Texas :)