I only started using Macs in 2005, though with hindsight I kind of wish I had ten, fifteen years before … But I want to know the answer to my own question now And it's kind of interesting — I took a blank canvas with a simple diagonal line from bottom left to top right, then applied 25 px Gaussian blur, and then changed that to 50 px. The simple line is 337 KB already, and that's without colour profile embedded (it was 1.2 MB with …), the 25 px blur is 610 KB, but the 50 px blur is 513 KB. I had not expected the file size to go down with more blur.

And then I tried saving the 50 px version as SVG, and the file size is only 96 KB …