They are not on average. Conventional cosmology predicts that statistically, galaxies faraway are different from galaxies close by. There was never the claim that there could be no outliers e.g. faraway galaxies that are much more massive than the average for their age.
In any case, there are thousands of papers describing the differences of galaxies close-by and galaxies far away. I gave three examples in this thread, reproduced here:
- The morphology of nearby galaxies are on average different from faraway galaxies ([astro-ph/0109358] The Morphological Evolution of Galaxies )
- The velocity dispersion of nearby galaxies are on average different from faraway galaxies ([1107.0972] Redshift Evolution of the Galaxy Velocity Dispersion Function )
- The star formation rate of nearby galaxies are on average different from faraway galaxies ([1207.6105] The Average Star Formation Histories of Galaxies in Dark Matter Halos from z=0-8 )
Not only galaxies, but observations of the Lyman-alpha forest shows that the entire universe evolves from being cold and neutral to being hot and ionized due to there being more stars and black holes once the universe is older.