This sounds like a "duh" rule but it's not. The two concepts work against each other and require different soft skills.
To increase your earning potential you need to free your time for activities that make more money- this often requires spending money (e.g. eating out to save time, paying to have your shirts pressed, etc). Reducing your expenses requires the elimination of excess, or being conservative with your existing resources… this also has a cost. It's a mindset that will keep you from making more money.
To put this another way: High earners are usually big spenders, and fiscally conservative people often have difficulty increasing how much they earn.
If there was only one rule to wealth creation: you need more coming in than going out. Being good at both is a paradox that you must master.