Antonym of MAGNANIMOUS

Magnanimous : generous or forgiving, especially towards a rival or less powerful person.
Selfish : (of a person, action, or motive) lacking consideration for other people; concerned chiefly with one's own personal profit or pleasure.
Naive : (of a person or action) showing a lack of experience, wisdom, or judgement.
Generous : showing a readiness to give more of something, especially money, than is strictly necessary or expected.
Small : of a size that is less than normal or usual.

Antonym of Magnanimous is Selfish