Soo... To get started, give the poor thing a longer body. The tail is in a somewhat unnatural position, so I changed that. Make the face fluffier, unless you're drawing a shorthair cat. Give it whiskers by it's eyes, and four toes. Oh, and a dewclaw. Cats also have whiskers by their nose, but I forgot to draw those.
On the shorter figure, the tail is also in a weird position. The back should be more straight than that, as I fixed in the picture. I didn't bother with the paws too much, as you can see, I didn't even draw toes on one of the paws. The chest and neck is good, I like it. Just make it a little less big.
Listen to the bird in the corner. He speaks the truth. He says:
"Follow the rules of anatomy... Or expect a visit from me..."
Bah, I just noticed that I forgot to draw a belly on the smaller cat. Will add.