Explain why, for Locke, extension is a primary quality?

Locke founded the idea that we perceive qualities differently. He argued that primary qualities such as extension, size, shape and weight do not change when we perceive them. They are mind independent, objective and public properties. They can be measured by instruments e.g. a ruler and will not change regardless whether I, Josh Steinert look at it or you Dr Green look at it. They will never change. For example I will always be 184cm tall regardless of who perceives me. Extension is like this because it can be measured. Secondary qualities are different. They change depending on who views them. For example the color I see on the desk in front of me may be different to the color you see belonging to the desk in front of me. Primary qualities cannot be separated from the object.