The direction of your effort for these two strokes should be completely different.
On the serve you hit up with a closed racquet face, while on the smash you hit down, opening the face of the racquet as neeeded, depending on the downward characteristics of the lob your are trying to smash and on your distance to the net.
Imagine holding a sizable rubber hammer and trying to blast the ball upwards, forward, and then pronating it to the right at the hit: that is your serve.
On a smash, you would get your hand close to the ball and then hit it downwards, flatter, and more forward than on the serve. Why?
Consider this: the ball has a pronounced downward trajectory, and you need somehow to be more coincident with that trajectory to find it better.
On the serve, depending on your toss, the ball is either stationed at the top of the toss when you hit it or going down at a reasonable speed. It is easier to find.