Can Irregular Menstruation Be Due To Uterus Size?

Hi, I am 20 years old. My menstrual cycles become irregular on and off in a year and sometimes they are regular for years together. I recently got an USG done showing ET uterus 6mm and size 8.3x3.5x4.9 cms. Though my doctor told its normal, I am little apprehensive. All my hormones are well within limits And I donot have anaemia.