I had cataract surgery a month ago, and now the operated eye looks smaller than the other one., what can I do to make eye normal again?

Droopy lid. Likely the eye is not smaller, just less open- or droopy. Wait at least 6 months, then have it evaluated by your eye surgeon or an oculo-plastic surgeon. Usually this is not an issue after both eyes have cataract surgery because they match, but it can be. Ptosis (droopy lid) does happen after cataract surgery; it is not necessarily serious. Be patient, then get evaluated.
Eyelid surgery. The eyelid can sometimes droop after cataract surgery because of the speculum that is used to keep the eye open during surgery. This gives the impression that the eye is smaller. A simple ptosis surgery can often correct this common finding.
Ptosis. The eye likely looks smaller due to drooping or ptosis of the eyelid on the side of surgery. It should get better with time but if it persists it can be repaired.
Wait. Sometimes the increased light now getting in can cause pseudo drooping of the lid that should adapt and equalize in time. Surgery to make them equal can always be done, but don't be in a hurry.The incidence of this condition has dropped enormously with modern cataract surgery.
??? Agree with doctor becker if it is due to eyelid appearance. If you are referring to the pupil, it could be that it will match the other eye once surgery is performed on it. It could be due to changes in the pupil from surgery also, which may persist. Ask your doctor.
Side effect of meds. It is common for the eyedrops (particularly steroid drops) given after cataract surgery to make the eyelid a bit droopy. Usually, the lid will return to its normal shape a month or two after you stop the drops. If it doesn't, check back with the operating surgeon.