also, boundaries tend to be marked by ropes, or those toblerone things, inside the fences - mainly I think to stop fielders injuring themselves chasing the balls. And I suspect that for T20 and 50-over matches they bring them in a little so more boundaries will be hit.
But grounds can only be as big as space permits. NZ ones like Eden Park are often rugby grounds. The MCG is enormous, 171 x 146 metres, but the new Modi stadium in India is 180 x 150 metres. A bit of willy-waving there.