The more mass something has, the more of both it experiences. When an object is sitting still, it resists moving, and the more mass it has the more it resists. The amount of thrust needed to move something and how fast it ends up moving are both directly tied to its mass. On the other hand, once something massive starts moving, it is very hard to stop. This is also due to the relationship between mass and inertia.
Gravity is also proportional to how much mass each thing has.
The bigger an object is, the larger the gravitational pull it exerts. Because of gravity and inertia, the more massive something is, the harder it is to get into space, the harder it is to keep it there, and the harder it is to move it where you want it to go when it is there. For that reason, one of the goals of the New Millennium project is to make lightweight spacecraft.