Yes this is normal geldings do not tend to fullly mature until 7 whereas mares and stallions mature around 4. This changes horse to horse it has something to do with them being cut when they are younger
The testosterone in stallions encourages the growth plates to close more quickly so geldings may well be bigger.
I was always told that cobs didn't finish growing til they were 7. My lad was 5 when I got him and is 2 inches bigger 2 years later-also a cob. Don't forget to take widthways into account, too. Because he wasn't much worked when I got him, he filled out sideways once in regular work so his rugs went from 5'6 to 6'3 in under 2 years!