Well more than heavier, it depends on the weight times distance (of each thing that makes it up).... the weight distribution and torque/moment to have it turn (while the fins align the whole thing from back end). Think of how a dart works if you throw it just generally etc.
So compared to the fins + motor at the back, there is also the long rocket body, the fuel tanks inside, the remaining fuel (that now settles so its heavy in the falling direction) etc etc. These make where the centre of gravity where it is.
Take a look at this, there is lot more than the motor and fin mass, i.e how casing, tanks and fuel are distributed (and unburnt fuel sloshed and settled in decline phase) etc: