I think the AT4 being a disposable weapon and the SMAW not being disposable can both be shown the same way. The M136 rockets are placeholders for the extra tubes, while the SMAW rockets are just themselves. All it doesn't model are the used tubes left on the ground, (or not left). That said, I wouldn't assume that just one AT4 should take the same amount of "inventory room" that 3 SMAW rockets take up. Don't SMAW rockets come in tubes anyways?