Old American people get bigger: why not? (3,2,4)
I believe the answer is:
may as well
'why not?' is the definition.
(I've seen this before)
'old american people get bigger' is the wordplay.
'old american people' becomes 'maya' (the Maya civilisation).
'get bigger' becomes 'swell' (I've seen this before).
'maya'+'swell'='MAY AS WELL'
(Other definitions for may as well that I've seen before include "for the whole of spring?" , "No reason not to" .)