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As we can see, in the first stage of Rocketlab's Electron the nozzles have small nozzle exit area.

Side view of Electron Rocket first stage's nine Rutherford engines Rear three-quarter view of Electron rocket second stage's single Rutherford engine

Source (all)

But for second stage why are they using nozzle skirt/ extension with huge exit area?

Comparison of Rocket Labs' Rutherford sea-level and vacuum optimised engines

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    $\begingroup$ More about the difference in the answers to: What are the differences between a standard Merlin engine and the Merlin Vacuum engine? $\endgroup$
    – uhoh
    Sep 21 '18 at 8:51
  • $\begingroup$ beautiful photos by the way, do you think you could add a link to the source? Thanks! $\endgroup$
    – uhoh
    Sep 21 '18 at 14:30
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    $\begingroup$ @uhoh The link is medium.com/@JackKelleher/… $\endgroup$
    – Amar
    Sep 22 '18 at 8:42
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    $\begingroup$ Thanks! btw add = "edit question and add it there" Comments should really be considered temporary, so for anything important or possibly helpful, it should be added to the original post (either answer or question). I've added it now to save you the trouble. $\endgroup$
    – uhoh
    Sep 22 '18 at 9:58