It might have to do with the low resolution of the background image. The small drops produce stronger distortion, compressing a large region of the image into a smaller area, while the large drops faithfully reproduce the blurriness of the image after it's already been blown up to the size of your screen. Everything looks more uniform if you load it in a smaller window, but if you go too small, you'll notice aliasing in the smaller drops.

I love this but the focus issue messes with my brain. Also snow on the ground and rain on the glass. Should be snow flakes stuck to the window. But I'm being picky now.

If you have good double-glazed windows, the snow doesn't usually stick. It'll build up in the corners, though... Or of there are screens over the windows it can stick to the screen.

(This focus effect is almost as if there is a screen, where the large drops run down the screen and the spattered mist is on the glass, and the edge of the depth of field is between the two...)