In the beginning of The Martian, the first MAV can reach orbiting Hermes just fine with 5 passengers on board. Yet towards the end of the movie, we are shown that the second MAV needs to lose weight to do the same, even with only one passenger. Why is this?
Shortly after Watney discovers he's stranded he mentions the Ares IV MAV is already on-planet, though it is far away - much further away than the buggy can take him. Needing to reach the Ares IV mission site (where the Ares IV MAV is already) is what prompts him to start retrofitting the buggy for longer journeys.
The MAV is an ascent vehicle, only designed to get its occupants into Mars orbit, and not much else. The Hermes should have been in Martian orbit when the Ares IV MAV launched, and the MAV would have no trouble reaching the ship.
However to rescue Watney the Hermes' return trajectory involved a slingshot around Mars, intentionally avoiding entering orbit so they'd be able to return home. This meant the Hermes was going significantly faster than the speed the MAV was designed to intercept at. In order for the MAV to gain enough velocity, its weight had to be reduced.