A LOT of small players dominate junior and do absolutely nothing in the NHL.
The problem more than size is usage.
In junior smaller players play in the top 6 and on the PP and get loads of TOI/g and opportunities and pile up the points. Their size not as much an issue when the age group is mainly 16-19, and all teams have smaller players, and physically immature.
However when they get to the pro level they get sheltered due to lack of size and playing against bigger, stronger and older/physically mature players and play on the 3rd and 4th lines.
In order for Petan junior numbers to mean anything, he would have to play in the top 6 and on the PP for the Leafs, but as depth player on the 4th line in limited minutes with lesser linemates its mostly ineffective based on different usage, and unlikely to find his game or make much of an impact beyond an injury substitute. Only the best smaller players make up the depth charts to scoring roles and then excel in the right situations.