1) Big companies have an enormous amount of friction and hassle that must be overcome to get things done. This is by necessity -- workers have to be coordinated with each other, and the more of them there are, the more overhead is required to accomplish that.
2) Big companies are boring to work for. Even when they're doing interesting things in the big picture, you're not likely to be working on the interesting bits. And when you are, it will be done in a way that makes it as tedious as possible.
I know many people with sensibilities similar to mine. I also know many people who strongly prefer to work for big companies. Big companies are just a different sort of beast, and if you enjoy riding it then more power to you. It's just not for everyone.