There are only 4 options here:
1) Either level 3 orcs are children, which is why they are three times smaller (therefore, orcs are idiots for sending underdeveloped children to war)
2) Or giant orcs are not orcs, but a separate race, like goblins (let's say, ogres)
3) Or orcs can grow into giant orcs through some artificial and complex method (magic, pharmacology, call it what you want)
4) Or orcs are a crazy species with huge size variability, but in that case, the height of the fighters is no longer 2 meters, but from 2 meters and up, which is a big difference with consequences for balance
Although, even with point 4, the average height of a fighter should still correspond to what is drawn, just like the leader, then we get even wilder proportions:
Orc height 1.5-3.5 m (if the height is 1.5-2.5 m, then these are fighters, if 2.5-3.5 m - leaders).