Icons are pixel images or vector images. Both have their pitfalls. Understanding these pitfalls will make you a better icon maker.

Icons used to be made as pixel images. Often the artist would make a 32x32 icon, and then scale it down to 16x16. Scaling the icon down makes it fuzzy, the artist had to repair it. Sometimes the 32x32 icon had to be simplified in order to make it possible to scale it down. Going back and forth between the icons the artist produced two icons that looked the same. Each time a new size is added a new icon has to be made. The number of icons in a set can become so big, the icon set becomes unmaintainable. Enter vectors.

Toolbar icons are very often concrete icons They are pictures or close representations of the operations which they represent. Toolbar icons are generally used much more than application icons. So one needs to find them and recognize the purpose they resemble fast. Therefore they are usually more symbolic and simple than application icons to improve usability. Toolbar icons are tools one just wants to use. Making them too detailed would decrease usability a lot.