What is a 'Common Size Financial Statement'

It displays all items as percentages of a common base figure. This type of financial statement allows for easy analysis between companies or between time periods of a company. The values on the common size statement are expressed as percentages of a statement component, such as revenue

Why commonsize matter?

Common-size income statements facilitate easy comparison. Not only can readers easily see how much of every dollar goes to rent, for example, they can compare that percentage to other companies or other periods in time. This allows analysts to compare companies of different sizes and not be "blinded" by the size differences inherent in the raw data.


sources:
http://www.investinganswers.com/financial-dictionary/financial-statement-analysis/common-size-income-statement-5237
http://www.investopedia.com/terms/c/commonsizefinancialstatement.asp