[jamoes]

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.