The decrease in size across the actinoid series is, element to element, greater than that across the lanthanoids. The reason generally given is that

1. A$5f$ electrons shield the nuclear charge even less effectively than $4f$ electrons
2. Bthe actinoids have fewer electrons in the outer shell
3. C$5f$ electrons shield the nuclear charge more effectively than $4f$ electrons
4. Dthe actinoids are all radioactive