Compared to a standard old-school 1913 railed handguard (which I'm pretty sure is the real question here) a KeyMod handguard with pic rail segments attached will be around the same overall external diameter, but that is taking an average because old style quad rails could vary quite a lot from brand to brand and model to model. However your KeyMod rail will 'look' fat by modern standards and because the areas with the rail attached will be comparatively a lot thicker then the rest of the handguard that's just plain KeyMod.
If you want to mount a laser or something along those lines which can only screw on to picatinny 20mm rail then just mount it on the 12 o/clock rail that's already there. Otherwise, as above, get KeyMod specific accessories. I believe Krytac KeyMod is cut to real spec so you'll want accessories from either real companies or PTS ideally. I don't know how well airsoft spec accessories attach to real spec handguards, no personal experience.