Airlines seats have been getting smaller as obesity has been rising
finance.yahoo.com
finance.yahoo.com
This article points out that passengers have waist sizes that are 2-inches larger than they were 20 years ago -- the timeframe over which seats and legroom have been getting smaller.
Spirit is the worst in this regard.
If you personally want a wider seat and don't mind the extra cost, you can have it. It's called first class.