[rarestia]

Because it’s falling down toward something very, very big?

Yes, it is being pulled further into the sun's gravity well. There's a very high dV requirement for inner system travel, because despite the speed gained by falling toward the sun, the object eventually needs to slow down or risk being flung out of the solar system. They are using several orbital braking maneuvers around Venus and Mars, IIRC.

[rarestia]

*Mercury