Why did Romanesque churches need thick walls and small windows?

In Romanesque architecture, churches required thick walls and few, small windows primarily to contain the significant outward thrust generated by heavy masonry vaults.

During this period, architects transitioned from timber construction to stone vaulting to improve fire resistance. Because these stone roofs exerted immense pressure on the building's structure, massive piers and thick walls were necessary to support the weight and prevent the walls from buckling outward. Consequently, the structural reliance on these solid, load-bearing walls limited the ability to incorporate large window openings, resulting in the characteristic dim interiors of the style.1