It keeps all the systems much simpler to deal with by forgoing inventory mass, and yes, instead represent that via the mass of the containers themselves. Notice that Storage Units are some of the heaviest parts in the game.
Worrying about the mass of individual inventory items on top of everything else would be quite annoying, and doesn't fit with my design philosophy that we should do away with tedium everywhere possible (which is almost everywhere, though still more improvements are coming in this area because I know there are a couple ever so slightly tedious aspects, especially in the late game).
Also note that no one in their right mind is walking around with 2700-time steps. When you see screenshots like that it's because their movement has been disabled during the current firefight without backups and it would be a really bad idea to even try to move
. (On a related note, there will soon be a feature whereby you can simultaneously detach all parts and run, if it comes to that.)