David Corenswet -- who's playing the Man of Steel in James Gunn's forthcoming 'Superman: Legacy' -- got snapped in a gym pic by fitness trainer Paolo Mascitti ... who's apparently working with DC to get him nice and hunky for the titular role.

Based on what we can see here ... their regimen is working, 'cause Dave's looking swoll!

You don't see the guy's face here -- but you can definitely get a sense of how his body's shaping up ... and he certainly looks a bit more Superman-ly now. It's a bit of a 180 from what we saw earlier this summer, when Dave hit the beach and bared his hot dad bod.

There was certainly nothing wrong with what he was working with then -- especially below the equator -- but let's just say ... he wasn't nearly as muscled out as he is right now.

Now that Dave seems to have the Clark Kent physique down ... it would appear he's checking all the boxes fans of the franchise want to see in their Superman. He definitely has the face and hair department on lock -- not to mention height -- and now ... he's ripped too.

Time will tell what he ultimately looks like in the blue and red suit ... but this is a sign things are headed in the right direction. Filming for 'Superman: Legacy' is set to get underway in Spring 2024 ... and the flick's being released in Summer 2025.