Your SVG imports at the wrong size? It is the DPI assumption.

You design a 100 mm part in Illustrator, export an SVG, and LightBurn or the Glowforge app shows 75 mm. Or the same file lands at a different size in every program you try. The artwork is fine. What differs is how many pixels per inch each program believes an SVG contains.

A pixel is not a size

An SVG stores geometry in user units. The file may declare a physical size in its width and height attributes, and when those say something like "120mm", every reader agrees on the size. But many exporters write pixel values there, or nothing at all. Then the physical size is pixels divided by an assumed pixels-per-inch value, and that assumption is where the programs part ways: Illustrator uses 72, the SVG specification and most current laser and CNC software use 96, and Inkscape releases before 0.92 used 90. Same file, three sizes, and none of them is "corrupt".

The factor is a fingerprint

Measure something in the imported file that you know the real size of, then divide. The result usually identifies the cause:

Illustrator: the "Responsive" checkbox

When Illustrator saves an SVG with "Responsive" enabled, it omits the width and height attributes so the graphic can stretch inside a web page layout. Sensible for websites, useless for cutting: the importer has to invent a size, and different importers invent different ones. The fix: File > Save As > SVG, open More Options, and uncheck Responsive, so fixed dimensions get written into the file.

Even with fixed dimensions, an Illustrator file measured in pixels can still land at 75% in a 96 dpi importer, because the pixels themselves carry the 72 dpi assumption. The workaround used in the Glowforge community: scale the artwork to 133.333% before exporting, so the shrink lands the geometry back at its intended size.

Inkscape: real units in the document

Current Inkscape works at 96 dpi, which matches what most laser software assumes. Set the document to real units in Document Properties (display units and page size in mm), draw at real size, and save as Plain SVG. Old files are the exception: anything made in Inkscape 0.91 or earlier carries the 90 dpi scale and comes in slightly small. Open such a file in a current Inkscape, check the document size in mm, and export it fresh.

Fix it at import

If you cannot change how the file was exported, correct the reading side. LightBurn lets you set which DPI it assumes for imported SVG files; look in the import section of its settings and match the value to the program that made the file. Failing that, scale after import: measure a known dimension and use the exact factor, 1.3333 for the 72-to-96 case, not an eyeballed "about a third bigger". On parts with joints or tabs, an eyeballed factor is a misfit you will cut twice.

What this is not

Wrong size at import is a declaration problem, not a geometry problem. Nothing needs redrawing, and no tool can "detect" the right size from the shapes alone: a 50 mm square and a 2 inch square are both just numbers in the file. Whoever fixes this, fixes it by making the declaration explicit. The same issue on the DXF side is covered in the DXF wrong size guide.