Humans aren't nearly as robust as ponies. Simple gestures such as hugs, pats on the back, and hoof bumps are all well and fine when it's another pony on the receiving end. With a person a fraction of their mass, ponies forget how fragile others are
The party is over, and everypony is going their separate ways. Pinkie at first falls into a deep depression, but through the help of the Cake family, gains a new perspective about the true meaning of friendship
While ponies around her grew and got their cutie marks, she remained the same. An encounter with a few neighborhood foals teaches her that, while she can't change on the outside, She can still be happy and make the most of who she is on the inside.