Keep in mind that Riot seems to be redefining what exactly a yordle looks like. It was stated that Ziggs and Lulu were the base for yordle aesthetics, the foundation. That does not mean either fit contemporary yordle standards. When Gnar was released, Riot explained they were closer to their vision of how a yordle should look, and apparently they concluded yordles should be larger in body, ears, and eyes.
Gnar has a fairly small body when compared to other yordles, even older Bandle City champions, with the exception of Rumble, but he is explicitly described as a child. In addition, Gnar is a primitive yordle, so his design likely plays a significant factor in deciding the appearance of modern yordles. For example, his eyes are massive, but the pupil dominates them. Tristana also has big eyes, and while they aren't quite so dilated as Gnar's, you can barely see her sclera. Gnar has ears nearly the same size as his head, while Tristana's are only slightly smaller than her skull.
Overall, Riot is moving away from "tiny, baby smurfs" for ladies and "walking hamsters" for men; no more heads with limbs attached. Compare Rumble's old splash with his new one:
http://whiterabbitcostuming.com/wp-content/uploads/2012/11/rumble-splash.jpg http://1.bp.blogspot.com/-k4jpbUNJvto/VDg2iyVK1pI/AAAAAAAAa90/9Vv1YRMTczE/s1600/Rumble_Splash_0.jpg
In the old artwork he had a body roughly the same size as his head with two strange trapezoids for ears. In the new image his body is proportionally larger and his ears are more triangular, not to mention his eyes are slightly larger.
While this may not be quite so credible, as in-game representation is somewhat skewed, Updated Tristana is only slightly shorter than Katarina.
In short, yordles are larger and more animalistic than previously conceived. To be completely honest, I like the change, perhaps not in its entirety, but I do appreciate the greater level of detail.
