Thanks so much everyone for your help!
Think about getting a weight tape and tracking height and weight on a monthly basis. It helps to see their progress month to month and you can identify growth spurts and adjust feed accordingly.
Also keep a close eye on joints for any signs of physitis like heat or enlarged joints. It’s very common in big, fast-growing foals and you need to nip it in the bud. Best to keep them lean, plenty of time to get them fat and slick when they are older.