I believe to remember the file was https://library.kiwix.org/viewer#wikipedia_fr_medicine_maxi_2024-06

With Firefox Responsive mode:

I have tried the following things:
open the link in Safari browser within the iPad simulator and the web inspector on the desktop.
It seems that with this combination the javascript "spell" we use: webkitTextSizeAdjust or the equivalent css: -webkit-text-size-adjust won't have any effect for some reason.
It seems that this is an iOS specific experimental setting:
https://developer.mozilla.org/en-US/docs/Web/CSS/text-size-adjust#browser_compatibility

The closest I could get is to bump up the font-size a bit, and observed the same outcome as reported in this issue.

By looking into the html/css part of the site:
It turned out that there's a media-query that centres the content:

• when it's full screen (side bar collapsed):

margin-left: auto;
margin-right: auto;
max-width: 55.8em;

• when it's 3/4 of the screen (side bar expanded):

margin: 0 16px;

What it means in practice is that by bumping up the font size (or text-size-adjust) the max-width: 55.8em is relative to the font-size applied, therefore it can "outgrow" the full screen width. (As em is a Relative unit in css: "em and rem are relative to the font size of the parent element and the root element, respectively.")
As an example changing it to (rem): max-width: 55.8rem; it preserves the margins.

Which brings me to the conclusion, that we are doing this zoom in a very unconventional way.
Although the proposed PR does fix this specific case, I think @rgaudin might be right that we should not fix specific content issues on the reader side.

I am a bit torn on this, and don't have a strong opinion if we should or should not fix it.
I am happy both ways of either merging the PR, until it's well documented we can apply it and revert if needed. Or throw the PR away and maybe in the future look into some alternatives we could use for "zoom in".

Postponing this issue