TYPO3 Core

hm can't really reproduce that, can you check out EXT:news which has a svg in tree as well (ext-news-folder-tree.svg)

The icon in EXT:news has already a viewbox with "0 0 16 16". Try a SVG image with another viewbox. Try the attached file.

The same behavior can also be observed in the category tree.
The icon I used here had a viewbox of 512x512.

The Icon-management of TYPO3 may cause the error, because it converts the original SVG-Tag into a G-Tag. The estimated viewBox will not work and the Icon will use the coordinates of the main SVG.
In contrast to HTML the SVG allow the Usage of the SVG-Tag in the body. Embeded SVG is a common technique, to solve many problems in a simple way.
[[https://stackoverflow.com/questions/5451135/embed-svg-in-svg]]

If I use a svg instead of g in the pagetree for the node-icon-container and add the original viewbox and set an correct transfer, then the icon has normal size.
(see screenshots)

I hope, the hints will help you to solve the problem. I tried your workaround, to convert the icons to 16*16-matrix by using embeded SVG's. But the following-construction failed:

<!-- This won't work as workaround. Embeded SVG are not correctly interpreted by the Icon-Registry of TYPO3 9.5.5 -->
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<svg
        xmlns:dc="http://purl.org/dc/elements/1.1/" 
        xmlns:cc="http://creativecommons.org/ns#" 
        xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" 
        xmlns:xlink="http://www.w3.org/1999/xlink" 
        xmlns="http://www.w3.org/2000/svg" 
        width="100%" 
        height="100%" 
        version="1.1" 
        viewBox="0 0 16 16" 
        preserveAspectRatio="xMinYMin meet" 
>
    <defs>
        <svg
                xmlns:dc="http://purl.org/dc/elements/1.1/" 
                xmlns:cc="http://creativecommons.org/ns#" 
                xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" 
                xmlns="http://www.w3.org/2000/svg" 
                xmlns:xlink="http://www.w3.org/1999/xlink" 
                id="myOriginal" 
                width="100%" 
                height="100%" 
                viewBox="0 0 64 64" 
                preserveAspectRatio="xMinYMin meet" 
        >
...
        </svg>
    </defs>
    <use xlink:href="#myOriginal" x="0" y="0" width="16" height="16"/>
</svg>

The rebuilding with the symbol-tag works, but the symbol-tag don't allow for example the usage of <LinearGradients> :-(

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<svg
        xmlns:dc="http://purl.org/dc/elements/1.1/" 
        xmlns:cc="http://creativecommons.org/ns#" 
        xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" 
        xmlns:xlink="http://www.w3.org/1999/xlink" 
        xmlns="http://www.w3.org/2000/svg" 
        width="100%" 
        height="100%" 
        version="1.1" 
        viewBox="0 0 16 16" 
        preserveAspectRatio="xMinYMin meet" 
>
    <defs>
        <symbol
                xmlns="http://www.w3.org/2000/svg" 
                xmlns:xlink="http://www.w3.org/1999/xlink" 
                id="myOriginal" 
                viewBox="0 0 64 64" 
                preserveAspectRatio="xMinYMin meet" 
        >
....
        </symbol>
    </defs>
    <use xlink:href="#myOriginal" x="0" y="0" width="16" height="16"/>
</svg>

It seems to me, that TYPO3 don't wrongly allow embeded SVG.
It was really hard to find this solution, because the caching was really unpredictable.

J. Peter M. Schuler wrote:

As suggested, editing the SVG is the most simple solution. However the viewbox is often quite hard to change nevertheless setting a transform resolves the problem easily.

Problem with transform:scale is, that the icons will be scaled in the "create Page" drawer also. Which makes them tiny or even impossible to see...:

I will try to find icons that orginally work with a viewBox of 16x16 Px. Seems to be the only feasible solution right now. :(
PS: Found a good converting tool which can resize your SVGs with the proper Viewbox dimensions: https://svg-converter.com/resize

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/69147

Applied in changeset 71aebd5f8cf8acd074a2f06c1ba7fbf96ef65cdc.