Chipworks concluded in an analysis of the 32 nm Apple A6 SoC that the six-transistor SRAM cell area was ~0.15 µm2. It estimates the same cell shrunk to the 28 nm process of the A7 would measure ~0.12 µm2. Thus it concludes the A7 sports ~1 Mbyte L2 cache and ~256 Kbyte L1 cache based on conservative 40-50 percent utilization to allow for the row and column circuitry.
But they bought MMI for the patents more so than for the hardware. Still, now that they find themselves in the hardware business, an SoC should not be automatically ruled out. There is always a cost/benefit analysis to be done, product differentiation, etc. It really depends on Google's strategic plan with that business, and how the relationship with its Android partners factors into that plan.
That would be madness IMO. Google should focus on what they are good at (software, search technology) and partner with others in other areas. Buying Motorola Mobile was a mistake in my opinion, so to go for the SoC design business would be madness for me....
Soc wouldn't be a good investment for Google, for that amount of money (few hundred millions at least) they can achieve amazing feasts in many areas, while a soc gets only a small relative and temporary advantage.
And anyways, they managed to get unique features in their soc in the motox phone by working with others, so it's seems good enough.
With the exception of law enforcement, I am not aware of anyone storing a full finger print image for verification. All that would be stored would be a short term image and 3 megabytes seems rather large.
For security, all that should be stored is a fingerprint signature. This is more than adequate for verification and removing false positives, but makes it pretty much impossible to recreate the fingerprint.