I has very small breasts and have been married for 6 years. Is it allowed in Islam to have enlargement surgery to please one's husband?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Enlarging the breast just to look more beautiful is unlawful because changing the creation that Allah has determined for the person is unlawful.  Allah forbids changing His Creation.  But if the breast is so small that it can be considered a defect that harms both the husband and his wife, then there is no objection to undergo the operation of enlargement just for this necessity.

Allah knows best.