I'm not wrong very often (just in case you're keeping track). It's true that there's chest involvement in the pullover, but, also significant back, involvement, too depending on how they're executed. You got your dig in, and you're right, to an extent. If the dumbbell is more to the sides, it'll involve more chest. When it's single, and in the center, it involves less of the chest. If you don't believe me, go do a few at the gym. I consider that it works both. I still don't consider it the best choice for "chest definition." I give you that chest is the prime mover, but, if you're laying across the bench, you'll notice it in your back, as well. As I said, I'm not usually wrong (that's why I'm me, and, you're not), but, I'll give you some on this one. You got me, but, failed to provide the solution for the original poster. I'm used to it and understand youthful ignorance. What were you saying about "douche bags" (quoting your profile)?
If you've done very many of them, you'll notice the pull in your back, big time.
At any rate, if you look at chest anatomy, or, know anything about definition, you know that definition is a function of body fat levels. A well executed fly would be much better for bringing up the chest. If you look at an anatomy chart, you'll see there's no lower or upper chest muscle.
What the original poster needs is a clean diet, and HIIT, to bring up definition. For sarcoplasmic hypterophy, he needs higher rep work. Period. Just the way it is. Pick an exercise. There's two chest muscle groups. Anything that works them will do but flies will likely be more direct, and effective.
I reckon that's why I have the 48 inch chest, and, you don't.