More than just the size, it is what you put into it
A big rage the past couple of years has been a proliferation of low cost 7' "parabolic" umbrellas from various manufacturers. I'm not going to get into the arguments of who created the first one, or who stole who's design. My concern is more with how they are being used.
By now we should all know that the size of the light relative to the subject determines the quality of the light. Large lights give a softer, less-shadowed look. Smaller lights give a harder, more contrasty look. So, the immediate thought about a 7' umbrella would logically be that it is a very soft light. And it is. Or at least it can be. But it also to depend on what light source you use with the big umbrella.
I see a few kits offered with the 7' umbrella and a bracket to use your speed light/lite with it. Speed lights are normally a small and hard light source. So, if you bounce it into a gigantic umbrella it should become a big and soft light. Makes sense, no?
Below is a set of images of such an umbrella with a speed light attached and bounced into the umbrella. The flash is a Canon 430EX II in manual mode. The three images across the top row were taken with this combo. In the first one, the flash was set to the 105mm zoom position. There is hardly filled any of the umbrella. Yes, the light will be larger and softer than direct flash. But probably not what you were expecting. So, for the second image the flash was zoomed out to the 24mm position. Uh-oh! That isn't all that much wider than the 105mm position when the umbrella is so close to the flash head. Additionally, we see that the flash bracket doesn't come close to pointing the flash into the center of the umbrella. What to do?
The third image was taken with the flash head's built in diffuser panel popped out and over the flash. It does a much better job of filling the umbrella, but I had to open up 3+ stops to get enough light into the umbrella. And it still doesn't quite fill the umbrella evenly, note how dark the bottom portion of the umbrella is.
I imagine that a bracket to hold 3 or 4 speed lights with the umbrella shaft centered around them would be a big help, but also a big expense. Unfortunately, I only have one speed light and a single bracket, so can't test that one out.
Just as a way of comparison, I took two more shots using a studio flash head, a Speedotron 202VF. The first of these images has the standard 7" umbrella reflector. Even with this combination, the umbrella isn't quite filled by light, but overall it is much more efficient. To get the umbrella to fill with light, for the last image I took the reflector off the strobe head. With the bare tube the umbrella is finally filled. It is not quite as bright. And you need to be a lot more careful if using the umbrella rig off to the side that direct light from the bare tube doesn't hit your subject causing very distracting shadows.
Take from this what you may. I just wanted to make sure you understand that just having a super large light modifier doesn't mean that you have a super large light source. I encourage you to do a similar test with your own lighting equipment to see what it is really doing.
PS: I also tested out a silver 7' parabolic. You can see those results in this post.
PPS: I then pulled out all the modifiers I could find around the studio and compared them all in action in a new post.