Desktop and Finder windows have separate settings. To shrink icons inside folders:
If you really want maximum information density, skip icon view entirely: how to make macbook icons smaller
Adjusting icon sizes on a MacBook is handled in different ways depending on where the icons are located (Desktop, Dock, or within Finder folders). 1. Resizing Desktop Icons To make the icons on your wallpaper smaller: Desktop and Finder windows have separate settings
For icons on your (not in Finder windows): how to make macbook icons smaller