that's because he's more powerful in mega gnar form and has range while in mini gnar form.
im not sure how outdated these numbers are, but they seem relatively close.
Mini Gnar gains 5.5% - 99% (based on level) bonus attack speed, on top of his base attack speed growth of 0.5%.
Mega Gnar gains 100 - 831 (based on level) base health, 3.5 - 38 (based on level) base armor, 3.5 - 29 (based on level) base magic resistance, and 6 - 48.5 (based on level) base attack damage.
edit
got excited so i forgot to make my final point.
fighters arent necessarily all built like garen or tryndamere. gnars fighter-esque thing is that he can be a marksman or a tank. it doesnt necessarily mean he is able to stay in combat a long time or that he is amazing at exchanging auto attacks. but what it does mean is that he's usually wanting to prolong combat. so you skirt the outside edge of a fight.
yes he's weaker than most, but he also has valuable tools other champs dont.