Why has my cheekbone become bigger than before? How can I make it like before?

I can't really say what you're referring to but the cheek bones themselves are not going to change with age. There may be some soft tissue changes that give the appearance. Try to post some good photographs for a better opinion.

It is unlikely that an otherwise healthy individual would grow bigger cheek bones as they age.  You may have gained weight and have chubbier face, or you may have lost weight and the drawn out face is exaggerating your cheek bones.  Pictures would be helpful.

Loss of volume is a significant factor in the aging face.  As we get older the skin, subcutaneous fatty tissue, and even our bones become thinner and lose volume.  It would be unusual for the cheekbones to become wider.  The cheek area may appear wider because of volume loss around the cheek. You may benefit from fillers, fat grafting, a facelift or other procedures.  Consultation with a board certified plastic surgeon will give you options that are best for you.