There have been different attempts of reforming the LOREG (Ley Orgánica del Régimen Electoral General). The last one was based on a report the State Council prepared for the Government. The State Council concluded that the electoral system is disproportioned and benefits the biggest parties and the least populated provinces, because it assures every province a minimum of two deputies in the Congress. The State Council proposed to enlarge the number of deputies from the present 350 to 400 and to assign only one deputy per province.
Here you have a more detailed explanation of this proposition:
This proposition was debated in the Congress last year and it was refused. The biggest parties and the nationalist and autonomous parties voted against this reform:
And this is complete text of the LOREG: