In specific regions they will require specific capabilities... some times they need a Frigate or Destroyer sized vessel to do a specific job and smaller vessels wont de even if they work out cheaper.
Certainly a long range corvette with a decent medium calibre gun (ie 76.2mm or 100mm) plus some 30mm gatling guns for close in defence against high speed boats and of course a helo would be ideal for an anti piracy mission as long as there was a support ship with it to keep it supplied with fuel and food and ammo and fresh water... it could operate for long periods off the coast of Africa the same as a Frigate with lower operating and manning costs as well as only taking a corvette out of the available fleet rather than a larger vessel.
Equally however if you want to send a vessel into harms way it needs decent air defence and attack capability which most corvette sized vessels lack.
Different basing locations require different defence and attack capabilities, which is clearly being reflected in the variety of small boats going into service.
With larger boats they can put more sensors and weapons on board so they can be more multirole and just one vessel type can be used to cover a much wider range of duties.
With UKSK launchers and Redut a wider selection of attack and SAM weapons can be carried and can be selected with the mission in mind.