The average length of a gtx 650 ti(not boost) is no longer then the pci-e slot, but the height is taller than most 520's and the card is 2 slots thick as well.
So they are NOT the same size(in a normal desktop case you should be able to fit one, but they are NOT low profile and take 2 slots in most cases).
You also run into even bigger GTX 650 ti's like this.