That raises the question of what exactly an indie game is and where do you draw the line between indie games and mainstream industry titles. There doesn't seem to be a clear definition of what makes an indie games developer.
A purely technical definition of independently developed games is that they have been made independently of a publisher. With this definition every game that is created and published by the same company is an indie game. This definition feels slightly too strict to capture the essence of what most consider to be indie games. There are examples of large "Triple A" titles that have been published by the developers that created them, especially in the mobile phone market where self publishing has become normal. There are also examples of games that have been released through a publisher but still have a very indie feel.
So if the technical definition doesn't feel like it quite fits the subject area that the term has come to cover, what can we use as a defining attribute?
The primary factor is that independent games are developed independently of any external pressures on the design of the game. The reason externally published titles often lose their indie feel is that the publisher puts pressure on the developer to alter their game design due to perceived market forces. This also applies to self published games that have their game design changed under pressure from factors other than what makes a good game. Even if a game is made by a lone developer, if the primary motivation is to "work an angle" or "cash in on a trend" then that isn't in the spirit of indie game development.
A second factor is that all of the team are engaged in creating a game that they themselves want to play. Games that are made by companies that are working a tried and tested process to earn a pay packet have a different feel to them. This is from an advert for a game designer for a large commercial game developer:
"use a variety of research techniques (e.g. surveys, observational studies) to better understand the needs, requirements and expectations of the target audience and the relative influence of industry trends/best practice".
In contrast, the designer of an indie game will usually just have a great idea that they want to turn into reality and know in their heart that it will be a great game without concern for market research or best practice.