Honestly, shrinking a portion or all of the human population doesn't sound like the worst idea. You can reduce the amount of resources humanity consumes as a whole or need to basis. Or you can focus on just shrinking high priority social security targets such as elderly to reduce their burden, or shrinking kids to reduce how much they need to eat or resources needed for their education until they grow up, etc.
The downside is there might be stratification and segregation between the big and small people
I'm not a scientist, but I have a feeling that some materials and things (like CPUs) don't work well if they're processed or even shrunk for a smaller size.
Granted, this whole scenario involves magic or some shit, and the existence of bringing sci-fi characters (and their technology) into reality might help with that.
And like the other guy said, even cats, chickens, and insects become dangerous if your small.
I used to care for chickens, and they will absolutely eat anything that's smaller than them. They were clucking that they found something to eat when they saw my kitten...