no heel…

Hi All, i just got a pair of use dboots (DC Phase) it fits very snug and nice when i’m just standing up but i noticed whenever i m on my toe side (heel up) i noticed it slips a little where my heel starts sliding up does this mean it’s too big? i tried half size down but it crushes my toes and i can’t stand it any comments/advices are appreciated, thanks a bunch in advance