i dont understand why are all the leds together and not divided into 4 cells like this led :
That's the common Cidly design. You'll see it used by a few companies. The modular nature makes it easy to replace components. Those are also 3w with reflectors and more comparble to TopLED's reflector series. (That company's use of the name BlackStar is confusing because Lighthouse Hydro makes a Blackstar series that aren't based upon Cidly's housing. But, they do make a "Chrome" series which appears to use Cidly.).
The downside to Cidly's modular design is that there are a limited number of bulb (spectrum) configurations that can be employed. For example, there seems to be a growing shift toward white light. With a single panel, 1 white LED could be placed in the center. Or, better yet, a cool and warm white LED could be finessed into the middle area. Cidly's repeating pattern of 15 LEDs forces you into 4 whites (or 8 if you wanted cool and warm).
I agree with you that smaller is better. I personally believe 4 smaller Mars IIs would be better (in a 4x4 tent), provide more diffused lighting (and height adjustment) than one large fixture. But, even that would allow for more spectral choice than the 15-LED modules.
There may be some improvement in store for the Cidly design. Grow Northern (who uses Cidly) is employing a new "rebel" module which uses 28 LEDs per module. I don't know how this is going to work out. It may be a huge improvement, or just trying to fix the limitations of 15-LED groups without fixing the root cause of that limitation. Doubling the number of LEDs in the same space seems like it would come with a negative consequence. But, I've read GN is powering them lower (which should cause the components to last longer. But... with the way technology changes, longer life may not be too useful. But, GN has offered upgrades to its customers which is primarily possible due to that Cidly/modular design. But.... so many factors to weigh!).