i might sound like a broken record here, but its all about anatomy.
what you have right now tells me that the two main issues you have are general facial anatomy knowledge, and understanding of how fat accumulates on top of the anatomy.
after you get those two resolved, you have to exaggerate certain feature to make it stylized. for example, scale the eyes up, make the nose smaller, etc. also the big ears are not really working out.
keep in mind that stylization without solid anatomy never work well. all good stylization artists can also create very realistic models if they needed to.
i actually found an old baby head bust that i sculpted a while ago, i tweaked some of the proportions to make it similar example to show you what i am talking about.
you can download a decimated version of the model here:
this model is not meant to represent your concept but only to show you how fat work on the face and when stylized.
here is a paintover with shots of the sample model:
some of the areas of focus:
concept. although you are working off of a concept it is ok to improve on it specially if the concept has some issues to begin with. for example, the ears on the concept are too big. unless she is supposed to be some sort of elf, the big ears dont work.
fat accumulation. right now you just scaled up certain areas without actually bulging it outward. for example, the crease that follows down the corner of the lips is too wide in your model. that would not happen when fat is pushing out from all around. she also appears to have a double chin which should work nicely to make her look fat.
curves. you need to make the silhouette more spherical in local areas. for example, think of the cheeks as a skin over a sphere shape pushing out from inside. same for the chin.
lips. when dealing with a chubby face like this the lips would rarely push out like that. only the edges of the lips should be prominent. right now it feels like her entire teeth set is pushing out her lips.