This is interesting. I'm not sure the mini is due for a new design, but given the move away from optical drives in 2011, the move toward PCI-E based SSD storage (m.2, formerly known as Next Gen Form Factor or NGFF) it may be significantly smaller in both size and power consumption. There is already a great deal of unused space in the minis.
But a new form factor may not happen until 2014 given cost (the mini is the low-cost entry mac) and 2.5mm hard drives and SSDs should remain cheaper than the m.2 (or custom Apple-connector) PCI-E based SSDs. Having said that, the iMac is already using the smaller blade SSD.