Stars are found in huge groups called galaxies. scientists estimate that the larger galaxies may contain as many as a trillion stars, while the smallest may have fewer than a million.
Why use "the" before a comparative adjective?
The implication here is that the scientists have established groups of galaxies based on their size: the group of smaller galaxies, the group of medium-sized galaxies [in which, presumably, we spin], and the group of larger galaxies. Together these three groups complete the universe. [I'm making up the examples; I'm not an astronomer - there may be other groups of micro and macro galaxies for all I know.]
He could easily have said "... that larger galaxies may contain..." but by using the article, he refers to a specific group.