I apologise if this has been asked before,but as the sensor of the nex range is the c size same as the alpha system, how can the lenses
be smaller? Presume it's to do with no mirror .

It's related to the register distance, and not only. The distance from the lens mount to the sensor in the NEX system is 18 mm while the A-mount is 44.5 mm.

It all means that the lenses would tend to be larger at the given same kind of design and while preserving the same grade of telecentricity.

But it also means that different kinds of design may be used efficiently to minimise the lens size. It may mean poor cheap lenses like the infamous SEL 16mm, or it may mean the best superzoom in existence as the SEL 18-200 really is, or it may mean the recent beautiful yet inexpensive Sigma EX DN designs.

All in all, the short register distance expands the lens design limits.