The earth rocket had a significantly larger payload than the lander. IIRC, the lander only carried a crew of ~3 people, whereas the earth rocket had to launch with a larger crew, life-support for an extended stay in space, and enough fuel to get to and from Jupiter.

edit: I forgot that they had another ship waiting for them in LEO already, so maybe the fuel point doesn't apply. But, it's still plausible that the earth rocket had to carry a much larger payload than the lander had to carry.

Conservation of fuel. Especially as the fuel for "classic" two stage rocket would be vastly cheaper than magic-fuel they used later in the movie.

Why everybody brings up this honestly stupidest ctiticism about the movie is beyond me. It's like you lack both imagination and understanding how space missions work.

Oh, don't mind that... How did they maintain acceleration around that planet near a black hole for 7 years? And who the hell would think that was a good idea? (Wouldn't it be passing under the ship nearly the speed of light, and thus every few seconds anyway?)