You raise a good point. Here's the reason (taken from a Toshiba White Paper) -
To avoid totally redesigning the HDD the afore-mentioned NAND controller and NAND memory need to be mounted onto the HDD circuit board. The conventional four-layer printed circuit board has been changed to a six-layer board allowing the components to be mounted on the same size board as that used by the standard HDD as shown in Figure 3.
The amount of space needed to mount the components was further reduced by adopting small sized DDR2 (rather than DDR1) for SDRAM. However, it was impossible to mount more than one NAND memory module.
With respect to NAND memory, single level cell (SLC) technology has ahigher cost of capacity per unit than multi level cell (MLC) technology, but also offers much higher performance. If the number of dies (NAND chips) is increased, the driveâ€™s performance will be improved considerably because of parallel processing. Since this Hybrid drive could not include more than one NAND memory, a cost/performance trade-off analysis was performed, which selected the 8-GB SLC (which uses four 2-GB dies).
So, just enough space for one NAND module.
Maybe, if/when Hybrid drives are designed from scratch we will see larger NAND caches. I feel it would make sense.