In a nutshell: most meditation practitioners sit with excessive lordosis (curve) in the lower back. This is partly due to our modern habits of using chairs, of not doing much physical labor that integrates the body, and so on. But it is also due to downright poor instruction that many teachers give, or to misunderstood instruction.
As an example of the latter, it is common to read in Zen texts that one should "push the hips forward." "Hips" is often used to translate the Japanese word "koshi." But this is a a misleading translation, since koshi refers to the entire lower trunk, not the hip joints alone (or, the iliac crests, which most folks seem to think are their hips). The result: many Zen students after reading such things "push their hips forward" by tipping their pelvises such that their lower backs curve excessively...precisely the opposite of what "push forward with the koshi" means (if you think of how the entire lower trunk engages when, say, pushing a heavy object you will catch the intended meaning). Many Zen teachers, in fact, advise their students to do precisely this.
Unfortunately, this causes the breath to be cut off at the solar plexus, since the diaphragm cannot move freely at all in that position. Because of this, the meditation depth and refinement cannot develop. Even after decades of sitting in such a way (something I have seen some folks do) there will be little transformation, since upper and lower body are not structurally integrated. A body like this will be unable to relax, and so cannot serve as the vessel within which the breath power may circulate. In other words: the samadhi condition cannot deeply manifest.
Playing into this somewhat, perhaps, is also the odd belief these days that one needs to increase the curve in one's back to remedy back pain, since "the back should have a natural S-curve." Of course, the back already has a natural curve without one trying to make it so....this is not a problem for modern people. Rather, the cause for much low back pain is precisely an excessive amount of this curve, leading to disc compression. All of this makes for good business for "lumbar pillow" makers, of course, but bad advice for people who are in pain.
Zen teachers who advise their students to arch their backs excessively have good intentions, I am certain. The damage done by such bad advice is not excusable, however.
In any case, the first photo shows a common meditation posture. The energetic feeling in this body is moving upward rather than grounded in the earth. Someone who sits this way will experience heat, tightness, or pressure in the head. The breath will be shallow. Thoughts will proliferate.
The second photo shows a natural back position. The body in this photo is settled, and can sit without difficulty for long periods because its structure is aligned. Nothing special is being done here, except that the student has allowed the low back and sacrum area to release, along with the stomach and solar plexus area. This body is integrated, and a deep, dynamic diaphragmatic breath can manifest.
If you practice zazen, please take a look at the alignment of your pelvis. And if you wish to put the above assertions to the test, I invite you to curve your low back as much as you can...and then try to take a deep breath to the belly.