Last year, Elon Musk outlined a bold plan to begin transporting large numbers of humans to the red planet in a concerted effort to begin colonizing Mars. It seems as though this plan was at once too big and too small, though. Rather than focusing efforts on landing a single capsule on the red planet, Musk wants to shoot for a plan that's more in line with his original grand vision of sending larger ships to establish a Martian colony.
But he clearly doesn't want it to be too big; no one can ever accuse Musk of not having a vision, but it also needs to be feasible and achievable if he ever actually wants to get SpaceX to Mars. That's where this tweet comes in. The original SpaceX plan involved a massive rocket with 42 engines, with a 12-meter diameter, that would ferry ships of a 100 people or more to the red planet. Musk is now hinting that the rocket he is planning to use will have a 9-meter diameter. According to Ars Technica, this could leave the rocket with half the engines (21) and, therefore, half the mass.
That's much more feasible on multiple levels: cost, mass (weight is the single most important criteria when considering spaceflight), technical complexity, and issues of production: Finding a place to build a rocket as massive as the one Musk had originally envisioned would be challenging. This 9-meter rocket would be much bigger than the 3.7-m Falcon 9s the company is currently producing, but as Musk points out, it would fit in their current factories. This is all a lot of conjecture, but considering the source is Musk himself, it's feasible that the company is building a smaller version of their original Interplanetary Transport System. And because this rocket could be easier to build (though still will be a challenge), it could bring Musk's Mars ambitions one step closer to reality.