The Type I Compact Flash card is 3.3mm thick. The Type II was created to be a little bigger at 5mm thick so that the storage capacity of the card could be increased by using micro sized hard drives (microdrives).
However, flash memory has now reached such large capacities and has caught up with the capacities offered by such small harddrives, that the need to use harddrives in this manner became redundant, and hence the Type II is generally no longer required within the market.
Compact Flash was a development of the PCMCIA card, these are the long cards for the slot on the side of laptops. Hence Compact flash Type I is the same thickness and width, just shorter.
The technical specifications of Compact Flash allow capacities of upto 137GB, and 64GB versions are readily available in 2010, so it is not long before this limit is reached. This may be the final nail in the Compact Flash Card’s coffin.
Some digital SLR cameras have switched to the smaller size SDHC card, such as the Nikon D80 for example. SDHC provides capacity upto 32GB and very fast transfer speeds equal to what Compact Flash can provide.
SDHC was a development of the original SD format, and introduced to overcome that format’s 2GB limit. The SDXC specification has already been released to manufacturers by the SD Association, this will allow capacities of up to a massive 2TB (2,000GB). This proves the SD format is being continually developed, whereas the Compact Flash format has remained relatively unchanged since its creation.
Due to the fact its only serious market is digital SLR cameras it is unlikely Compact Flash will be developed past the 128GB level, especially since SD is used in many other devices than just Digital SLR Cameras, and these camera are already adopting the SD format.