...If I do that on Photoshop, I'll get a huge white page that at 100% goes way out my screen...

Because Photoshop is a raster imaging program. It deals with pixels. It's rulers count pixels. 100% on Photoshop's rulers means 1:1 ratio. That's one image pixel to each monitor pixel. At 100% or greater in Photoshop, you are not looking at interpolated pixels (i.e.; pixels downsampled to display on your monitor).

In Photoshop, set your rulers to Inches. Create a new file measuring 1" x 1" at 72 PPI. Create another new file measuring 1" x 1" at 300 PPI. Tile the windows, zoom both to 100% and you'll see that "an inch at 100%" in the first is smaller than "an inch at 100%" in the other. That's because in both cases Photoshop is showing you all of the pixels contained in an inch at 100%.

In other words, Photoshop is all about the individual pixels of the single raster image that your file will eventually be flattened to. So its rulers are all about image pixels. (In fact, Photoshop's rulers used to more correctly display "1:1" where it now displays "100%". (Another example of things being changed at repeated requests of users who don't know what's good for them.)

... if I start a document with the same size in Illustrator, I get a page that's smaller than my screen (at 100%)...

Because Illustrator is a vector drawing/design program. It deals with objects, not pixels. Its rulers measure real-world measure. 100% on Illustrator's rulers means 100% of the size of your artwork when printed, based on Illustrator's assumption that one monitor pixel represents one point (i.e.; 1/72 inch) of real-world measure.

In other words, Illustrator is all about individual, free-floating objects. Some of those objects are vector paths. Some of them are raster images. Unlike in Photoshop, each of the raster images can be independently scaled to different PPI. So it makes no sense whatsoever to think of Illustrator's rulers as counting pixels.

Do not expect Illustrator to act like Photoshop. You'll just confuse yourself. They are two very different kinds of programs corresponding to the two primary (and very different) kinds of computer graphics: Raster images and vector paths.

When trying to understand programs like Photoshop and programs like Illustrator, the understanding is in their differences, not their similarities.

JET

ansiaaa wrote:

 

... you said that at 100% Illustrator represents one real life point as one pixel on the screen...,

No Illustrator doesn't and can't show one point or any other unit on screen being equal to 1 point or any other unit in real life on monitors that are available on the market. JET added "based on Illustrator's assumption that one monitor pixel represents one point (i.e.; 1/72 inch) of real-world measure."

So, Illustrator rulers will show units equal real life units only if your monitor pixels are with the size that fit 72 of them in an inch. You can't find such monitor on the market now, they were available in the 90s.

to achieve what you want (100% in both programs to give the same size image on screen), in Illustrator create documents with pixel units and a number of pixels that equal the number of pixels in the Photoshop document. However if you right click on the rulers and set them to other units like inches for example, then the size in other units in the two programs will not match unless the image in Photoshop is set to 72 ppi.

ansiaaa wrote:

...at 100% Illustrator represents one real life point as one pixel on the screen, but that's exactly what any graphic software does...

That is a wrong statement. There is no program that does this. At 100% Photoshop does not show you the real size units. Just right click on the ruler and choose inches for example and then take a real life ruler and measure your screen, you will see that they do not match. To make the ruler in Photoshop equals real size ruler, you have to choose View > Print Size and the result will be correct if you have entered the pixels per inch of your monitor in Screen Resolution field in the Units and Rulers category of the Preferences.

Illustrator doesn't have such feature.

ansiaaa wrote:

 

...

But as I said, the same page setup (Print / A4 / 300DPI), in Illustrator ...

Illustrator doesn't know what 300ppi is and you don't have such option when creating a document or vector objects. PPI in Illustrator can be used only when dealing with raster images or raster effects and each raster image in a document can be with different ppi.