Myopic patient uses concave lenses,you might have read in physics that any thing looks smaller if you see through concave lenses.I think ,your power is increasing& whenever you change the glasses&some power increases,your eyes will look smaller.Your eyes are not becoming smaller at the age of 18 yrs,in most of the patients at this age chances of increase in myopic poweris very high.

Hello....... This change is because of wearing spectacles.. You can undergo surgery called lasix.. Or else you can also wear contact lense.. But you have not mentioned your power, repost query with type and power of eye sight... Consult ophthalmologist in nearest hospital.....