The most obvious disadvantage is financial. Another less obvious problem is the amount of quality time that a parent can spend with each individual child. Other than fun/quality time available, the amount of supervision & instruction time must be divided between multiple children. Alone time for the parents is severely restricted as well. Physically passing illness from one person in the family to the other is often a problem...smaller family means less duration for illness as a whole. That is some of the disadvantages, there are also benefits but then you did not ask about that.
