French SSNs are indeed the smallest SSN in the world, but at what cost?
Even if the Amethyste refit on the original Ruby class improved their stealth abilities, they still are far less stealth than British or US counterparts. They also are among the slowest SSN in the world, maybe the slowest. And if their size allow them to be perfect SSN for Mediterranean and littoral operations, as seen during the Kosovo war in 1999, they also have a very poor weapon load with only 14 weapons.
But the major problem is that the subs are so small that even if they can't carry enough food for more than 45 days in normal mission...
The new Barracuda/Suffren class will correct most of this problems. The Amethyste refit of the Ruby class already improved their stealthiness, but the new class will benefit from all the experience of our new SSBN class. The ships will also be longer and therefor faster underwater with a better operational endurance, but will still be "thin" ships, allowing the Marine Nationale to keep its expertise over Med and littoral combat operations.
The main problems for this new design is that we were cheap when it comes to carry heavy loads of weapons. The subs will only carry 21 weapons, not very better than the Amethyste when you take in consideration than we'll had the MDCN cruise missile in inventory (and of course no VLS...)
And the ships will still cost the same price than a Virginia Class, overall (actually the Virginia is probably cheaper), and no one can say that we French don't know how to build cost-efficient ships!
Of course, that's mainly because we'll only have 6 ships, when the Navy already ordered 16 Virginia subs...
But what I wanted to say is that it won't be necessary cheaper to develop, build and commissioned a new smallest class of SSN in the US. A small class of small ships could cost as much as larger ships from a larger class, moreover if we're talking about SSN.
The only reason I could see the US building a smallest class of subs is if they really want to export them, they could buy some SSK in order to reassure potential customers (that's what we did with the Lafayette stealth frigates, that's kinda what US and French did with F-104 and Mirage 2000-5 etc.)