https://history.house.gov/Institution/Apportionment/Apportionment/
If the state population doesn't include illegal immigrants then the total population of that state is lower. In states like California with a very large illegal population that equates to fewer seats in the House. Since the number of members in the house stays the same overall, that means other states with growing legal populations pick up more seats.
The Democrats recognize that not counting illegals primarily hurts states that have large numbers of Democrat held seats in Congress so they want this rule squashed.
Right, Texas is notorious for having too many democratic seats.