You guys often ask me how come my cake toppers don't sink even though they have such large heads ;) Below you'll find some TIPS:
Let the body dry before you attach the head. It needs to be very firm. If you attach the head when it’s not firm enough, the body will sink under the weight of the heavy head.
Make the head flattish. That way the head will be large but less heavy (than if it was round) and it won’t lean to the front or back.
Let the head dry before you attach it. It needs to be very firm. If you attach the head when it’s not firm enough, it might start sinking or leaning to the sides.
If you are using fondant (instead of gum paste or modelling paste), add CMC powder (tylo powder/tylose powder) to it. Add it shortly before forming the parts. CMC makes the paste thicker and speeds-up the drying time. If you don't add CMC, the fondant will be soft and the cake topper will sink.
Build the cake topper on a styrofoam dummy. The stick that goes through the body and into the head, should reach the bottom of the dummy. The longer the stick, the better it will secure the topper in the cake. If you leave the stick long and trim it to the depth of the cake later, it will provide support (the topper won’t sink). Additionally you can use dowels (f.ex. plastic straws) if there’s need.