Well you have to consider Kutta–Joukowski which is:
Force/Width = Circulation * Velocity * Density
the velocity and the airdensity obviously doesnt matter in this case so well leave that out.
This leaves us with
Force/With ~ Circulation
which means that, as the underbody usually isn't completely taken up by the venturi, since theres the driver, the engine and lots of other useless heavy bits, that need to be seated low, like wishbones, making the aero guys life miserable, the width or span of the profile tends to be smaller than that of a wing, which can easily cover the entire width of the car.
So to get the same amount of force the circulation needs to be larger.
However there's a difference.
Wings usually feed from air, that goes over the car, and pull (more so than push) it upwards to create downforce.
Venturis on the other hand take the air, that produces the force, from the frontal area of the car, that would otherwise, on its own, push that air away, creating a vacuum at the tail. So for one you cut away quite a bit of the front of the car and let that air stream under the car and come out at the back at road level, right where the trailing cars front wing/venturi-intake would be.
What's more is, that often the diffusers of venturis are built in such a way, that the hot exhaust gases from the engine feed into them. From my understanding, the idea here is to energise and thus speed up the otherwise intentionally slow stream at the diffusor, which then, through equilibrium conditions, forces the air in the venturis slimmest bits to speed up even more; sort of like the way you can make stars explode by collapsing their photons waveforms millions of years later.