Lulu's ultimate, after an initial knockup, gives the target a slow aura and increased maximum health (shown as the target being bigger). However, since the target is bigger, their attacks should have more destructive force due to there being more mass involved in their attacks.
To understand where this is coming from, you need to understand that destructive force is based off of the speed of an object colliding with something, as well as the mass of the object colliding with something. For example, if you are having trouble knocking pins down in a bowling ally, you can either send the ball down the lane faster, or you could use a heavier ball. The buff essentially takes the second option.
I mean the weapons are bigger, so they are bound to have more mass. A bigger sword swing at the same speed as a smaller one will deal more damage. As will an axe, a bullet, an arrow, and so on. That said, physics says that Lulu's ult should give a damage buff to basic attacks.