Its complicated.
I would say contentment would be a secret to simplicity. I don't mean contentment that limits either, because I also believe that if you have the propensity to be greater, to have more (to bless more), to not do so is to some degree theft. What I mean, when speaking of contentment, is to not live your life based upon the green pastures of others. Envy and jealousy only breed bitterness.
Rather, be content where you are, but continue to grow. Contentment that plateaus progress becomes stagnant. In other words, don't keep up with the Joneses instead be individualistic. Know who you are, the desires of your heart, and walk in the will of God because when you walk in your purpose you're most fulfilled.
So, how do we keep things simple? Contentment that is not a limitation. A proper perspective on the things of life, and the things to come. This doesn't mean that you can't enjoy the finer things of life, it simply means that you understand their insignificance in the big scheme of things. You don't let things have you, you have them. Love people and use money, and never the other way around. Concepts like these allow us to simplify life in terms of loving others, and loving God. A matter of priorities.
Simplifying your life doesn't necessitate downsizing, its coming to a full understanding with wisdom the proper placement of God, people, and things in your life. Looking at the intent of your surroundings such as a nice house, a nice car, and a beautiful family. Why do you want these things? Are they filling a void that really should be addressed in another way? Are they simply things you enjoy, irrespective of other's opinions? Look at your heart, be honest.
You can have these things, but seek first the Kingdom of God and His righteousness. Simplicity under these terms then is to have contentment with a proper perspective, acknowledging priorities [or keeping them in mind]. To enjoy life, but more importantly to share it. To love.