The difference in payload mass between MSL Curiosity and InSight isn't just because of the landing mechanisms, but the launch vehicles. MSL used an Atlas V 541, while InSight used an Atlas V 401.
From looking on the NASA performance website, it looks like the 541 has about twice the usable payload to Mars (I think the C3 for a Earth-Mars transfer is about 16 km2·s-2, not 100% sure).
The Atlas V 3-digit variants are the (1) fairing size, either 4 or 5 meters in diameter, (2) the number of strap-on SRBs, 0 to 5, and (3) the number of second-stage engines 1 or 2, though through the end of 2018 there has never been a dual engine second stage (why?).
Other launchers are plotted for comparison; you may also notice that the Falcon Heavy which is current launcher with the greatest thrust, is barely competitive with the Atlas V 541 for Mars missions; this is due to the lack of a cryogenic, high-efficiency upper stage. Scott Manley (of KSP fame) talked about this in a video a while back.