Keep sheet fit separate from screen fit
A smaller on-screen drawing should not mean a smaller craft.
The two sizes you see
The responsive preview fits your screen. The SVG file uses millimetre width/height and a matching coordinate viewBox. That file sizing is separate from how large the browser displays the preview. A downstream application can still rescale an SVG; check its import and output settings.
A worked A4 budget
A4 portrait is 210×297 mm. With 10 mm margins and a reserved 15 mm footer, the available net area is 190×262 mm. The 150×120 mm example fits without scaling or rotating the net. A 240×120 mm net does not fit that portrait width, even though it may look small on screen.
The fit report compares the unrotated envelope with that usable area. It also checks the swapped envelope and may suggest swapping base length and width. It does not silently rotate, shrink or tile the template. Portrait/landscape changes the sheet orientation explicitly.
Choose an export
Sheet SVG includes the selected paper size and a 40 mm scale-check line in its footer. Its download is disabled when the net does not fit. Net-only SVG uses the envelope plus your margin and is available even when the chosen paper is too small. Use appropriate larger stock rather than assuming a fitted page is full scale.
SVG reference: MDN viewBox documentation.