Microwave ovens, like conventional ovens, cook from the outside in.
If you have a large block (e.g. a cake), the microwaves are unable to penetrate to the centre - they only heat the outside edge.
Because of the efficiency and speed that food cooks in a microwave oven, there is insufficient time for the heat to be conducted into the centre of the food, so it stays raw.
If you want to cook a bulky item in a microwave, then you should set the oven to 'low' or 'defrost' power, and operate it for a correspondingly longer period of time - this will allow time for the heat to reach the centre and cook it satisfactorily without burning the outside.