The solar panels on the Phoenix Lander and InSight rover seem to take on an iconic new look of circular solar panels. As far as I could see solar panels on landers and rover took up the maximum surface area that they could be packaged into. For Pathfinder, that meant three triangular panels, for its accompanied rover, Sojourner, that was a simple rectangle on its top.
The Mars Exploration Rovers has a slightly more difficult design of Solar Panels, but ended up with a similar triangular wing shaped design to that of Pathfinder (as it used a similar tetrahedral lander to Pathfinder, I believe).
However, with Phoenix (2007) and InSight (2018) a circular solar panel arrangement has been used for whichever reasons. A similar design can also be seen Enhanced Cygnus spacecraft, as well as some mock-ups of the Orion spacecraft, where it is almost expected (by amateurs/ the casual public) to have rectangularsolar arrays.
I'm curious as to what led to the decision of the use of circular arrays in the Phoenix Lander and InSight Rover as opposed to trying to pack in as much solar array area as possible.
NB: I'm marginally aware that the Enhanced Cyngus uses circular panels to reduce its weight and increase its max payload by ~700kg 1