SpaceX Dragon Maximum Return Payload

For one of my university courses, we were asked to assess the viability of an asteroid exploitation mission. Assuming that we can somehow mine and refine the material (platinum in this case), we still have to bring it back to Earth (I know that it would probably be more viable to use the mined materials directly into space, but the work clearly stated that we have to bring it back).

According to this, the SpaceX Dragon capsule has the highest return payload from the ISS, (2.5 tons for pressurised cargo). Nonetheless, it has a pressurised volume of 10 m$^3$ (the Dragon wikipedia page actually states a return payload, of pressurised cargo, of three tons). Assuming a platinum density of $\rho = 21.45$ g/cm$^3$, the volume of the dragon would allow for a maximum mass of 214.5 tons.

That being said, I was wondering from where does the mass constrain arises. For launch it is understandable (you'd need substantially more fuel). But for re-entry, the only implication I can think of is in the ballistic coefficient, and I'd wager that we could change the spacecraft to meet the re-entry requirements. The only other problem that I can think of is that we have to put it on a re-entry trajectory, and need fuel for that. Still, if we somehow overcome that step (in our case, the spacecraft is coming directly from the asteroid so, as far re-entry is concerned, I don't have to worry about that constraint), is there something else limiting the mass?

Since this is a bit text, I'll summarise:

• From where does the maximum return payload on the Dragon spacecraft arises?
• Possible hypothesis: fuel to put the spacecraft into the re-entry trajectory; change to the ballistic coefficient;

If you could at least point me to some sources, I'd appreciate it =)