Because inexplicably, there's random pixel-level noise baked into the blue area. You can't see it unless you crank up contrast, but it makes the bitmap hard to compress losslessly. If you remove it using threshold blur, it doesn't change the appearance at all, but the size is down to 100 kB. Scale it down to a more reasonable size and you're down to 50 kB.
Because inexplicably, there's random pixel-level noise baked into the blue area. You can't see it unless you crank up contrast, but it makes the bitmap hard to compress losslessly. If you remove it using threshold blur, it doesn't change the appearance at all, but the size is down to 100 kB. Scale it down to a more reasonable size and you're down to 50 kB.
Modern web development never ceases to amaze me.