Baking is a science. Let's break down the ingredients and examine how they mix and mingle
BUTTER
Fact: More butter means a wider spread and a softer cookie
Butter is what helps to connect the proteins and provide a ton of flavor for your cookies. The higher ratio of butter to other ingredients, the softer the cookie will be. More butter means a wider spread and ultimately, a larger cookie.
EGGS
Fact: Extra egg whites = taller cookies -- Extra egg yolks = fudgier cookies
Egg whites provide water and protein to your recipe. The more egg white used in your cookie that taller it will be. Bedsides butter, eggs are the primary source of protein.
Alternately, egg yolks are fattier and can help enhance the soft texture and will make it feel more like a brownie than a cookie.
SUGAR
Different types of sugar used with yield a different type of cookie – white sugar is more likely to retain moisture while brown sugar is more flavorful.
LEAVENING
Baking soda and baking power are used to infuse oxygen into your cookies, and which is used is a matter of personal preference. When tested, baking powder was found to yield more cake like cookies that are taller with smooth and shiny tops. Baking soda produced denser cookies.
FLOUR
Since flour provides the bulk of the structure in a cookie, the amount you use can alter the texture of the cookie. A small amount of flour compared to butter (a ratio of 1 to 1 or less) will give you cookies that spread out into a wafer-like lace cookie. Extra flour (a ratio of 1.3 to 1 or higher) will give you cookies that barely spread at all as they bake, with centers that stay dense and dough-like, even after being almost fully cooked.
Flour makes up the bulk of the shape of the cookie and the amount used can change the texture and look. Less flour will make cookies thinner whereas extra flour will help keep the center doughy.
CHOCOLATE
Most chocolate chip cookies use, guess what, chocolate chips. They’re easier to distribute and are more even in your dough.
Read the full breakdown at Good Eats