Two wikis which are ostensibly setup the same way have different results when it comes to thumbnail quality and the sizes available by default.
Neither site had
$wgSVGConverters set so I tried setting that to ImageMagick on B with no change in results.
But there are two problems (which are probably related): quality; and the default set of thumbnails created. Whereas A offers
Size of this PNG preview of this SVG file: 512 × 512 pixels. Other resolutions: 240 × 240 pixels | 480 × 480 pixels | 600 × 600 pixels | 768 × 768 pixels | 1,024 × 1,024 pixels.
B only offers a couple resolutions.