NFmin is the minimum noise figure achievable for a particular device.

As a device, from any technology, is scaled up the transconductance tends to increase (and input referred noise decreases) and the input thermal noise due to resistance goes down, so it makes some sense that NFmin will reduce as the width increases from the minimum. But, of course, at some point the increased input capacitance is so high that the effective transconductance starts to decrease with increasing area, so NFmin will start to increase again at high frequencies.

Hope this helps a little.

I simply meant that as the device area gets bigger, so does Cgs which reduces ft (i.e. high frequency gain is limited by the capacitance). As the gain reduces, then the input referred noise increases, so NFmin increases with increasing frequency.