I looked over the model in lolking, and zed is fine.
I think you may be mistaking his "bulky" look due to his clothing. the baggy pants and armor overall. If you take the time to look over the model in lolking at all the angles, his proportions are perfectly fine.
When you look at the Zed model in lolking, and slowly turn the model in the original in game angle when playing, you will notice the reason for his bulky thigh/leg look is simply due to the "Red Drapery" coming down from his waist and wrapping around one leg, making it look "bulky/fat" when its not, its simply more layering of clothing.
I would also like to note also that any in game model character you look at in league of legend is at a up above view, causing a lot of distortion in proportions also known as "Foreshortening".
In conclusion Zed is not fat, and rather its the layering of clothing causing the bulky look, Hope this help and happy gaming!