Please don't overfeed your foal in some misguided attempt to get it to grow faster or bigger than it naturally should. You will risk causing developmental issues including bones that grow too fast for the tendons. It's just not worth the risk.
I had a field at the bottom of the moors where they mined for iron ore, the water ran orange there was that much iron in it. Everything grew bigger than it should and it worried me to death that we would end up with OCD or similar issues. The only saving grace was that they didnt seem to do it any faster than normal, they just got bigger! You really dont want to force feed a foal for growth.
I was always told that a colt that was gelded would grow to be taller than if he had been left entire. One of my foals had to be gelded at 4 days old due to a hernia. He finally stopped growing at 17.3 (his mum was 16.2 and his dad was 16.3).
Back to the OP though - why on earth would you want to encourage a foal to grow faster?
i would imagine it is how the hormones change with maturity. I bought a 6 month old foal in 1994 that had to be gelded due to a hernia. I thought it would stunt his growth and I wanted a big horse. Dam 16.2. Sire 16.2 pure ID's. The foal grew like a weed and was huge at 2. Finished at 18h.