Don't mistake the word 'lesser' to mean 'weaker', a "lesser god" is merely a god that does not hold dominion over the world, and does not influence Acolytes to such greater degrees.
The followers of these gods have to be extremely dedicated to them, in order to follow them effectively.
These gods can in fact be more powerful than either a Minor or Major God, depending on the circumstances. Abstergo- the God of the Void, for example, is capable of destroying every atom in existence, undoing everything, and returning the world to Void. He does not however, hold dominion over the world, and Acolytes of his are extremely rare.