Your filly is NICE!
I find the heights & genetics quite interesting.
When I started with Shetlands, one of the things I was looking to do was cross our Shetland stallion (45.25" or 11.1 hh) on larger horse mares for Shetland X ponies. When we started buying mares - I tried to find mares that were smaller themselves (ease of breeding) but had some larger horses behind them. Did that with the Hackney pony mare (she was 12.2 sire/dam both over 13 & 4 full sisters matured to 12.1 or 12.2 w/ the 1st one being permanent at 11.1 7/8") and with 2 arab x mares and with 3 purebred Arab mares (also had one we never bred). With the one mare, I predicted her first foal for us (2nd for her) would be right around her height of 14 - 14.1 hh. Everyone said no, wouldn't happen. That particular colt matured taller than her (
almost didn't make his pony height measurements!). On the others, I predicted that the foal(s) of the one mare would be closer to 13.2 (both fillies were) and on the other wasn't sure... the mare was VERY short, but rather stout. That pony has finally finished and I need to get an official measurement - his dam was 13.3 and I think he's 13.2 or 3 hh.
Our 2nd Shetland stallion (barely 40" at the withers) has been the one with all the surprises. Mostly, even out of larger mares w/ larger breeding behind them, his foals are smaller. If he himself was a mini, I guess he'd be called a size reducer, LOL.
Our 3rd and 4th stallions we don't know yet. 2015 & 2016 are their first foals. I do plan on doing some measurements on them now.
You might find this article interesting.
Predict horse height
With our first stallion - we did follow this type of measuring only we used it on yearlings and older only. I DID find that the measurements were accurate when the pony was mature. I kept up with that for years - that first Arab X "defied" the measurements until he was older before he sprung up taller than his dam... I didn't mention it above because I couldn't remember if you measured all the way down to the coronet band or if you stopped at the middle of the fetlock (true cannon measurement) and then was looking for this type of article since I no longer have some of my original ones (or my notes from those years).
Now I'm really curious. I will measure the fillies I still own by our 2nd stallion over this weekend and see what they measure. The youngest will be 4 yrs old on April 2nd. The oldest will be 8yrs old on April 21st. She is "small" by my idea and is 1/2 Shetland our of a 13 hh Arab/Hackney pony mare. Her two full brothers both grew much larger, though I don't have mature heights on them...
We'll also measure the 2016 fillies!