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.
What is that noise actually? It's clearly not JPEG artifacts. Is it dithering from converting from a higher bitdepth source? There do appear to be very subtle gradients.
I'd bet that it's AI generated, resulting in the funky noise.
Make it an SVG and it's down to 1kb.
None of this is due to "modern web development". It's just about a dev not checking reasonable asset size before deploying/compiling, that has happened in web, game-dev, desktop apps, server containers, etc. etc.
This should be an SVG (a few kb after proper compression) or if properly made as a PNG it'd probably be in 20-ish kb.