The Vinaya contains 227 rules for male monks and 311 rules for female monks. Is it known why so many additional rules were put in place for Bhikkhunis?
Thus the Bhikkhunī Pāṭimokkha contains 85 rules for which there are no direct correspondences in the rules for the bhikkhus. Some writers have interpreted these added rules as sign of an attempt to oppress the bhikkhunīs unfairly, but it should be noted that:
more than one third of these extra rules were formulated to protect bhikkhunīs from being the direct recipients of the abusive or careless behavior of other bhikkhunīs;
two of the extra rules (Pācittiyas 6 and 44) prevent bhikkhunīs from putting themselves in a position of servitude to bhikkhus or lay people;
according to the rules' origin stories, all but three of the extra rules (Pācittiyas 59, 94, and 95) were formulated only after bhikkhunīs complained to the bhikkhus about an errant bhikkhunī's behavior.
Tellingly, these last three exceptions were formulated after complaints initiated by the bhikkhus, and they touch directly on the formal subordination of the Bhikkhunī Community to the Bhikkhu Community. However, they are counterbalanced by two rules exclusive to the Bhikkhu Pāṭimokkha — NP 4 & 17 — that were formulated at the request of bhikkhunīs to prevent bhikkhus from abusing their position in the hierarchy in a way that would interfere with the bhikkhunīs' practice of the Dhamma. For a more detailed discussion of the checks and balances in the relationships between the two Communities, see The Buddhist Monastic Code, volume II, chapter 23.
For what it's worth there is a further set of opinions, published here: Extra rules. This too says that the rules "primarily dealt with the protection of the nuns".
In the booklet "Women in Buddhism - Questions and Answers" by Chatsumarn Kabilsingh Ph.D., this question is answered in some detail with a comparison of the monks and nuns Pāṭimokkhas. This quote shows where a large amount of the difference lies:
In Patidesaniya section, there are eight rules for bhikkhunis. Bhikkhus have the same content of the rules but they are counted as one and classified under Sekhiya, another section. This is one of the reasons responsible for the bloated number of rules for bhikkhunis.
In the Pacittiya section, bhikkhunis follow 166 rules as compared to 92 for bhikkhus. There are 70 common rules shared by both Sanghas. Then bhikkhus have another set of 22 exclusively for bhikkhus and bhikkhunis have another set of 76 rules exclusively for bhikkhunis. Within 76 rules exclusively for bhikkhunis, it is notable that there are many rules regarding ordination requirements which the bhikkhus also have to follow, but for bhikkhus they are not counted in the Patimokkha. This also results in a seemingly larger number of Patimokkha rules for bhikkhunis.
Due to the above reasons, the bhikkhunis carry a higher number of rules in the Patimokkha than the monks, but in reality they follow a similar set of rules.