I don't get what you mean though, they could be any size in this picture. He could even make Pinkie bigger than she is now, that would only mean she'd be closer to the 'camera'.
You do know about physics right? Pinkie is a few meters away from the 'viewer' while the mountains are several dozens of kilometers away. That's called a '3D environment' and is not a perspective issue.
The perspective of the background itself IS off though... But the ponies' size is fine.
There ARE numerous perspective issues in this image, but Pinkie is not one of them... She could be ANY size as long as she's larger than Derpy. She's a relatively short distance away from Derpy and her size is about the same, possibly a bit smaller. So I don't see the problem.
Either way, comparing Pinkie's size to the size of the mountains is, in this case, a pretty dumb thing, as Pinkie is in the foreground and the mountains are several dozens of miles in the background...